Use this plain-language glossary to understand common eye anatomy, refractive-error, procedure, diagnostic, and lens terms. It is educational information, not a substitute for an examination or individualized medical advice.
The clear front surface of the eye that provides much of its focusing power.
The cornea is the clear, curved front surface of the eye. It bends incoming light and supplies much of the eye's focusing power before light reaches the natural lens and retina. LASIK, PRK, and several other refractive procedures work by changing corneal shape. Its thickness, curvature, surface health, and overall structure are therefore important parts of a refractive-surgery evaluation.
The thin outer cell layer that protects the cornea.
The corneal epithelium is the cornea's thin outermost cell layer. It helps create a smooth optical surface and acts as a protective barrier. It is handled differently in different procedures: LASIK creates a flap that includes the surface layer, while PRK removes the epithelium so it can heal over the treated surface. Surface health can also affect comfort, tear-film stability, and the reliability of preoperative measurements.
The main supportive layer within the cornea.
The corneal stroma is the thick middle layer of the cornea. Its organized collagen structure helps the cornea remain clear and maintain its shape. In LASIK, laser reshaping is performed in stromal tissue beneath the flap. Surgeons consider stromal thickness, the planned treatment, and the amount of tissue left afterward when deciding whether a corneal procedure is appropriate for a particular eye.
The colored ring of tissue that controls pupil size.
The iris is the colored ring of tissue around the pupil. It changes the pupil's size to regulate how much light enters the eye, much like an adjustable camera aperture. In a refractive-surgery evaluation, pupil behavior can be one part of understanding visual symptoms in dim conditions. The iris itself is not reshaped during LASIK or PRK, which act on the cornea.
The natural lens inside the eye that helps focus light.
The crystalline lens is the natural, flexible lens located behind the iris. Together with the cornea, it focuses light on the retina. Its ability to change focus gradually declines with age, contributing to presbyopia. The lens can also become cloudy, forming a cataract. LASIK reshapes the cornea and leaves the natural lens in place, whereas lens-based procedures replace or add an intraocular lens.
The central retinal area responsible for detailed central vision.
The macula is a small central area of the retina that supports detailed, straight-ahead vision used for tasks such as reading and recognizing faces. It is located at the back of the eye, not on the cornea. LASIK changes how light is focused onto the retina, but it does not treat macular disease. Retinal health is part of an eye examination when vision symptoms need explanation.
The nerve pathway that carries visual signals from the eye to the brain.
The optic nerve carries visual information from the retina to the brain. Vision depends on both a clear optical pathway and a healthy optic nerve. Corneal refractive procedures can reduce dependence on glasses or contacts, but they do not treat optic-nerve disease. A comprehensive eye examination may include assessment of the optic nerve when symptoms or risk factors suggest it is needed.
The opening in the center of the iris that lets light enter the eye.
The pupil is the dark opening in the center of the iris through which light enters the eye. It becomes smaller in bright light and larger in dim light. Because a larger pupil uses more of the eye's optical system, pupil size can be considered alongside corneal measurements and visual goals during refractive planning. It is one factor among many, rather than a stand-alone measure of candidacy.
The light-sensitive tissue lining the back of the eye.
The retina is the light-sensitive tissue lining the back of the eye. It receives focused light and begins the process of turning it into visual signals. LASIK and PRK reshape the cornea so light can focus differently on the retina; they do not treat retinal conditions. Retinal health, especially the macula, is relevant when blurry vision cannot be explained by a refractive error alone.
The tough white outer covering of the eye.
The sclera is the tough white outer layer of the eye. It helps protect the eye and maintain its overall shape. It continues into the clear cornea at the front of the eye. Although LASIK is a corneal procedure, the sclera is relevant to the eye's broader anatomy and to other forms of eye surgery that may involve the outer wall of the eye.
The thin layer of tears that coats and lubricates the eye's surface.
The tear film is the thin layer of tears that coats the front of the eye. It helps keep the ocular surface comfortable and optically smooth. An unstable tear film can cause fluctuating blur and can make corneal or wavefront measurements less repeatable. Dry-eye evaluation and treatment may be part of refractive-surgery planning, because surface comfort and visual quality both matter before and after a procedure.
The clear gel-like material that fills the main cavity of the eye.
The vitreous is the clear, gel-like material that fills the large space between the lens and the retina. It helps the eye keep its shape and is part of the eye's internal anatomy. LASIK does not operate inside this space; it changes the cornea at the front of the eye. New flashes, a shower of floaters, or a curtain-like loss of vision need prompt clinical attention rather than assumptions about refractive error.
A refractive error in which the eye focuses light unevenly.
Astigmatism is a refractive error in which light does not focus evenly at a single point. It often relates to uneven curvature of the cornea, though the natural lens can also contribute. It can cause blur or distortion at more than one viewing distance. Glasses, contact lenses, corneal laser procedures, and certain intraocular lenses may be used to address astigmatism, depending on the eye and the treatment plan.
Farsightedness: a refractive error that can make nearby vision difficult.
Hyperopia, often called farsightedness, is a refractive error in which the eye has too little focusing power for its length. Light tends to focus behind the retina without corrective lenses. Depending on age and the amount of hyperopia, both near and distance tasks can be affected. LASIK or PRK may be options for selected eyes, but suitability depends on a complete examination rather than the prescription alone.
Nearsightedness: a refractive error that commonly blurs distant vision.
Myopia, often called nearsightedness, is a refractive error in which the eye has too much focusing power for its length. Light tends to focus in front of the retina without correction, so distant objects may appear blurred. Glasses, contact lenses, and some refractive procedures can change how the eye focuses. The right option depends on prescription stability, corneal findings, eye health, and personal visual priorities.
Age-related loss of the natural lens's ability to focus up close.
Presbyopia is the gradual age-related loss of the natural lens's ability to change focus for near tasks. It is different from myopia, hyperopia, and astigmatism, though it can occur alongside them. Reading glasses, multifocal contacts, monovision, or lens-based options may be discussed depending on a person's goals. Standard LASIK does not restore the natural lens's flexibility, so near-vision planning should be part of the consultation.
Surgery that removes a clouded natural lens and replaces it with an intraocular lens.
Cataract surgery removes a natural lens that has become cloudy and replaces it with an intraocular lens (IOL). It treats lens clouding rather than reshaping the cornea. Lens selection can be planned around refractive needs and visual priorities, but results and glasses needs vary by eye and lens choice. Read full article →
A broad term for LASIK planned with individualized measurements.
Custom LASIK is a broad, non-uniform term for LASIK planned with individualized measurements, such as wavefront or corneal-topography data. The exact treatment profile and useful measurements vary by platform and eye. It does not promise perfect vision or mean one approach suits every person; a surgeon evaluates the quality of the measurements and the rest of the examination. Read full article →
A procedure that places an implantable lens inside the eye to correct refractive error.
ICL is short for implantable collamer lens, a type of phakic intraocular lens placed inside the eye while the natural lens remains in place. It can be considered for selected people who are not suitable for corneal laser correction or who have other refractive needs. Because it is intraocular surgery, screening and follow-up differ from LASIK. Read full article →
A corneal laser procedure that changes the eye's focusing power.
LASIK is a corneal refractive procedure that uses a flap and laser reshaping to change how the eye focuses light. It can be considered for certain refractive errors in appropriately screened eyes. The procedure leaves the natural lens in place, so it does not treat cataracts or stop age-related near-focus changes. Candidacy and expected vision depend on the complete eye examination. Read full article →
A surface laser procedure that reshapes the cornea without creating a LASIK flap.
PRK, or photorefractive keratectomy, is a laser vision-correction procedure that reshapes the corneal surface after the epithelium is removed. Unlike LASIK, it does not create a corneal flap. Healing and early vision recovery follow a different pattern from LASIK, and the best procedure depends on corneal findings, prescription, ocular-surface health, and personal needs. Read full article →
Lens-replacement surgery performed to address refractive goals before cataract treatment is the main need.
Refractive lens exchange, or RLE, removes the natural lens and replaces it with an intraocular lens to address refractive goals. It uses a lens-replacement approach similar to cataract surgery, but the reason for surgery differs when a cataract is not the primary issue. It may be discussed when lens aging and presbyopia are important factors. Read full article →
A small-incision laser procedure that removes a shaped piece of corneal tissue.
SMILE, or small incision lenticule extraction, is a laser refractive procedure that creates and removes a small lens-shaped piece of corneal tissue through a small incision. It does not use a LASIK-style flap. Its availability, supported prescriptions, recovery, and suitability differ from other procedures, so the choice should follow careful measurements and a discussion of goals. Read full article →
LASIK planned with a wavefront measurement of the eye's optical system.
Wavefront-guided LASIK uses a wavefront measurement to add optical information to the treatment plan beyond a standard glasses prescription. The measurement may describe some higher-order aberrations when the scan is reliable. It is one possible customized approach, not a guarantee of a particular visual result or a substitute for corneal and ocular-surface evaluation. Read full article →
The controlled removal of a small amount of corneal tissue by a laser.
In corneal laser surgery, ablation means the controlled removal of a small amount of corneal tissue to change its shape. An excimer laser performs this reshaping in LASIK and PRK. The treatment pattern is planned from measurements and the intended refractive correction. How much tissue can be safely removed is individualized and depends on corneal structure, not on a single rule or number.
A thin hinged layer of corneal tissue created during LASIK.
A corneal flap is a thin, hinged layer of corneal tissue created during LASIK. The surgeon lifts it to access deeper corneal tissue for laser reshaping and then repositions it. A flap may be made with a femtosecond laser or a microkeratome, depending on the technique. PRK and SMILE use different approaches and do not create a traditional LASIK flap.
A possible additional refractive procedure used to refine a prior correction.
An enhancement, sometimes called a touch-up, is an additional procedure considered when a prior refractive result leaves or develops a meaningful residual prescription. It is not automatic after LASIK. Before recommending one, a surgeon evaluates healing, prescription stability, corneal measurements, ocular-surface health, and whether enough tissue remains for a safe plan. Read full article →
A laser used to reshape corneal tissue during procedures such as LASIK and PRK.
An excimer laser is the laser commonly used to reshape corneal tissue in LASIK and PRK. It delivers energy in a controlled pattern that removes tissue from the planned area. The aim is to change corneal curvature so the eye focuses light differently. It is distinct from a femtosecond laser, which may be used to create a LASIK flap or the tissue planes used in SMILE.
A very short-pulse laser used for precise tissue separation in eye surgery.
A femtosecond laser delivers extremely short pulses that can create precise tissue separations. In LASIK, it may be used to create the corneal flap. In SMILE, it creates the tissue planes and small incision used to remove the lenticule. It is not the same as the excimer laser used for corneal ablation in LASIK or PRK. Read full article →
A mechanical instrument that can create a LASIK corneal flap.
A microkeratome is a mechanical surgical instrument that can create the corneal flap used in LASIK. It is an alternative to a femtosecond laser for this part of the procedure. The equipment, surgeon experience, corneal anatomy, and the practice's available technology all influence how flap creation is discussed. A microkeratome does not perform the excimer-laser reshaping itself. Read full article →
The supporting corneal stromal tissue left beneath a LASIK flap after treatment.
The residual stromal bed is the supporting corneal stromal tissue left beneath a LASIK flap after the planned reshaping. It is one part of a surgeon's assessment of whether LASIK is appropriate and whether any future enhancement could be considered. The calculation uses several eye-specific measurements and clinical judgment; it should not be reduced to a self-screening threshold or a promise of safety.
An intraocular lens design intended to provide a broader continuous range of focus.
An extended depth of focus, or EDOF, lens is an intraocular lens design intended to provide a broader continuous range of vision than a conventional single-focus lens. It may be discussed during cataract surgery or refractive lens exchange. The amount of useful near, intermediate, and distance vision and the possible optical tradeoffs vary by lens design and person. A surgeon can explain how a specific option fits your priorities.
An artificial lens placed inside the eye to replace or supplement focusing power.
An intraocular lens, or IOL, is an artificial lens placed inside the eye. In cataract surgery and refractive lens exchange, it replaces the natural lens after removal. In phakic-lens procedures such as ICL, an implanted lens is added while the natural lens remains. IOL selection can involve distance, near, astigmatism, and lifestyle priorities, and no single lens design is universally best for every eye.
An intraocular lens designed with one primary focal distance.
A monofocal IOL is an intraocular lens designed with one primary focal distance, often selected for distance vision but also usable for other targets. Glasses may still be needed for tasks outside that planned range. Some people discuss monovision with monofocal IOLs, using different focus targets in each eye, but adaptation and suitability vary. Lens choice should follow a conversation about visual tasks and ocular health.
An intraocular lens designed to provide more than one focal range.
A multifocal IOL is an intraocular lens designed to provide more than one focal range, often with the goal of reducing dependence on glasses at different distances. The optical design can involve tradeoffs, including how light is distributed and how some people experience vision in dim conditions. It is not appropriate for every eye or every visual priority. A cataract surgeon reviews eye health, testing, and expectations before recommending a lens type.
A common cataract-surgery technique that uses ultrasound energy to break up the lens.
Phacoemulsification is a common cataract-surgery technique in which ultrasound energy breaks the cloudy natural lens into small pieces so it can be removed through a small incision. The surgeon then places an intraocular lens in the eye. The term describes a surgical step, not a particular lens choice or visual outcome. Your surgeon can explain how the technique and IOL plan apply to your individual eye.
An intraocular lens designed to help address corneal astigmatism.
A toric IOL is an intraocular lens designed to help address corneal astigmatism at the time of cataract surgery or refractive lens exchange. The lens must be selected and aligned for the individual's measured corneal astigmatism. It may reduce astigmatism-related dependence on glasses, but results depend on measurements, healing, lens position, and other eye factors. It is one of several possible components of a refractive plan.
An instrument that measures how light travels through the eye's optical system.
An aberrometer is an instrument that measures how light travels through the eye's optical system. It produces a wavefront measurement that can describe standard refractive error and some higher-order aberrations. Measurement quality matters: tear-film instability, pupil conditions, fixation, and other factors can affect repeatability. Surgeons interpret an aberrometer reading together with refraction, corneal imaging, and the rest of the eye examination.
A map of the cornea's surface curvature and shape.
Corneal topography is imaging that maps the curvature and shape of the cornea's front surface. It can help identify regular and irregular patterns and supports screening and treatment planning for refractive procedures. It is different from a wavefront measurement, which describes the eye's overall optical behavior. Topography is interpreted with corneal thickness, refraction, ocular-surface findings, and clinical examination rather than on its own.
The unit used to describe the focusing power of a lens or refractive correction.
A diopter is the unit used to describe the focusing power of glasses, contact lenses, and refractive corrections. A prescription may use plus or minus diopters and may include a separate cylinder value for astigmatism. The number helps describe refractive error, but it does not by itself determine candidacy, procedure choice, or expected outcome. Corneal measurements and overall eye health are equally important in planning.
Subtle optical irregularities not fully described by a standard glasses prescription.
Higher-order aberrations are subtle optical irregularities not fully described by the sphere and cylinder values in a standard glasses prescription. Examples include patterns called coma and spherical aberration. Everyone has some degree of these imperfections, and their practical effect varies. A wavefront measurement may detect them, but a detected value alone does not mean treatment is necessary. Surgeons consider repeatability, symptoms, and the complete exam.
Measurement of corneal thickness.
Pachymetry is the measurement of corneal thickness. It may be performed with ultrasound or optical imaging as part of a comprehensive eye evaluation. In LASIK planning, corneal thickness is considered with the planned correction, corneal shape, and tissue structure to assess safety and suitability. A thickness measurement should not be interpreted as a pass-or-fail number without the other clinical findings.
A condition in which tears do not adequately support eye comfort or surface health.
Dry eye syndrome describes symptoms and surface changes that occur when tears do not adequately lubricate and protect the eye. It can cause burning, grittiness, fluctuating vision, or discomfort, but symptoms and causes vary. Because tear-film health affects measurements and healing, dry eye is assessed before refractive surgery and managed as needed. Read about LASIK candidacy with dry eyes → Read about dry eyes after LASIK →
A strategy that sets one eye for distance and the other for nearer tasks.
Monovision is a vision strategy in which one eye is corrected more for distance and the other more for nearer tasks. It may be discussed with contact lenses, LASIK, or lens-based surgery when presbyopia is a concern. Some people adapt comfortably while others do not prefer the balance between eyes. A contact-lens trial may help explore the experience before a permanent surgical plan is chosen.