Glossary of Eye Care Terms - M-O
macula - The most sensitive part of the central retina, responsible for visual acuity and color vision.
macular degeneration - Disorder characterized by changes in the eye's macula that result in the gradual loss of central vision. The exact cause is unknown, but appears to be related to a genetic predisposition, smoking and several other risk factors. Central vision may be blurred, distorted (metamorphopsia) or shadowy before vision loss occurs. Please click here to learn more about age-related macular degeneration.
macular edema - Swelling of the central portion of the retina (macula), due to buildup of fluid leaking from retinal blood vessels. Causes temporary or permanent vision loss if untreated.
macular hole - Hole in the eye's macula; many doctors believe it can be caused by vitreous shrinkage as we age. Symptoms include blurring or a blind spot in central vision and metamorphopsia. Read our article on macular holes.
maculopathy - Any disease of the macula, the most sensitive portion of the central retina responsible for detailed vision and color perception. One example is age-related macular degeneration.
madarosis - Eyelash or eyebrow loss. Causes include infections, metabolic disorders, blepharitis, certain drugs, lupus erythematosus and trauma.
mast cells - Histamine-containing cells that are part of the body's immune system and play a key role in the inflammatory process. Mast cells are located in tissues throughout the body, including the conjunctiva of the eye and eyelids.
medial rectus muscle - Muscle that moves the eye toward the nose.
median - When applied to numbers, a value that falls exactly in the middle of a specified range. Half of the numbers are above the median, and half of the numbers are below the median. As an example, "five" is the median of a range of numbers from one to nine.
meibomian gland - Gland found in the eyelid that produces the oily outer layer of the three-layer tear film that lubricates the eye.
meibomianitis - Inflammation of the meibomian glands; rosacea is a common cause. Symptoms include red or pink eyelid margins, a red or pink eye, dryness, burning, blurred vision and a swollen eye.
melanin - Pigment that colors the iris of the eye as well as other parts of the body, including skin and hair.
melanosis - Condition characterized by melanin (pigment) deposits in the skin or eyes.
meningitis - Inflammation of the meninges, membranes that envelope the brain and spinal cord. Viruses and bacteria can cause meningitis. Symptoms include fever, headache, vomiting, a stiff neck and light sensitivity.
meridian - One of a number of radially arranged imaginary lines, each of which passes through the center of the pupil when viewing the eye head-on. Generally separated in one-degree increments, meridians are used to determine the location of the most- and least-curved sections of the cornea when prescribing lenses with cylinder power to correct astigmatism. Meridians are also used to describe the shape of corrective lenses.
metamorphopsia - Vision problem in which objects appear distorted. For example, straight lines may appear to be wavy, curved or bent, objects may appear to be larger or smaller than they actually are, or closer or farther away than they actually are. Metamorphopsia is typically caused by conditions or diseases that affect the eye's macula and retina.
microaneurysm - A weakened area in the walls of tiny blood vessels. In diabetic retinopathy, microaneurysms can occur in the retina from damage related to abnormally high blood sugar levels. As microaneurysms in tiny blood vessels (capillaries) expand, ruptures can result. These ruptures lead to hampered blood flow as well as swelling and leakage that sometimes cause scarring, blind spots and blindness.
microcornea - Abnormally small cornea.
microkeratome - Small instrument that surgeons use to cut the cornea.
microphthalmia - Congenital defect resulting in an abnormally small eye or eyes. The cause is usually unknown. Microphthalmia typically results in blindness or reduced vision, but normal vision is possible if the eyes are nearly normal in size.
migraine - Severe headache, sometimes accompanied by nausea and visual disturbances. Visual disturbances alone are also possible; this problem is called an ophthalmic migraine, or migraine without headache. Eye and vision symptoms include blurred vision, ptosis, halos around lights, light flashes, light sensitivity, eye pain or discomfort, vision loss (blind spots in central vision, tunnel vision or overall impaired vision), distorted vision and wavy lines in vision.
milia - Raised tiny white bumps on the surface of the skin, often appearing on the eyelid and around the eyes and nose. Milia occur when dead skin cells do not slough off properly and become trapped at the base of a sweat gland or hair follicle, forming a small keratin cyst. They are commonly seen in newborns, but also can affect adults of all ages.
minification - Making objects appear smaller; the opposite of magnification. Most lenses for nearsightedness make objects look smaller, and when the lenses are in eyeglasses, they also make the wearer's eyes look smaller. Aspheric lenses reduce this minification effect, for a more natural look.
monochromatic - Refers to one wavelength of light, as opposed to the many wavelengths of light found in varying colors.
monofocal - Type of spectacle lens, intraocular lens (IOL) or contact lens design that has only one corrective lens power. A multifocal lens has more than one lens power, enabling sight at multiple distances (focal points), typically for people with presbyopia.
monovision - Vision correction method for those with presbyopia in which one eye is corrected for near vision and the other for far, either through contact lenses or refractive surgery. Monovision eliminates the need for reading glasses, but does have some drawbacks, including decreased depth perception. Read our article about monovision with contact lenses.
mucin - Lubricant such as saliva that protects body surfaces. In the eyes, mucin is a tear layer that helps moisten and protect the eye's surface.
mucormycosis - Fungal infection typically occurring in the sinuses or lungs and mainly acquired by those with compromised immune systems and by diabetics. Symptoms include sinusitis, eye and facial pain, fever, a bulging eye and vision loss.
multifocal - Type of spectacle lens, intraocular lens (IOL) or contact lens design that includes more than one area through which the eye focuses. such as bifocals or trifocals. Examples are bifocals or trifocals. This enables sight at multiple distances, typically for people with presbyopia.
myasthenia gravis - Weakness of the voluntary muscles, believed to be autoimmune in nature. Symptoms include double vision and eyelid ptosis; patients sometimes have non-eye symptoms as well, such as difficulty swallowing or using the arms and legs.
myelin - A sheath made of proteins that covers nerve fibers. Myelin is essential to transmission of nerve impulses carrying information to and from various parts of the body. When myelin is destroyed or damaged in the optic nerve the result is optic neuritis, with vision loss or distortions.
myokymia - Common eyelid twitch typically brought on by stress or fatigue.
myopia - A refractive error where a person can see near objects clearly but distant vision is blurred. Myopia typically begins in childhood and progresses during the school years. The primary symptom of nearsightedness is squinting. Also called nearsightedness. Learn more about myopia.
nanometer - A measurement of length equal to one-billionth of a meter. Abbreviated as nm. Wavelengths of light are measured in nanometers. Eyeglass and sunglass lens manufacturers use nanometers to describe the different types of light that may pass through or be blocked by a lens, whether it be visible light, ultraviolet light, blue light, etc. For example, visible light has a wavelength range of 400 to 700 nm. The amount of light transmission itself is described as a percentage. For example, a dark sunglass lens might allow only 12 percent of visible light to pass through to the eye.
nearsightedness - See definition for myopia.
neovascularization - Abnormal growth of new blood vessels, such as in an excessive amount, or in tissue that normally does not contain them, or of a different kind than is usual in that tissue. Also see angiogenesis.
neuronal ceroid lipofuscinosis - Rare, hereditary, degenerative disease in which the body does not store pigments called lipofuscins properly; the disease is characterized by vision loss, seizures and dementia. Types include Santavuori-Haltia (infantile), Jansky-Bielschowsky (late infantile), Spielmeyer-Vogt (juvenile) and Kufs' disease (adult). Some types may also be called Batten disease.
neurons - Nerve cells that form intricate networks through which sensory information is transmitted via electrical impulses sent throughout the body. Neurons are part of the central and peripheral nervous systems, which support functions such as thought, sight, perception, hearing, taste, feeling, speech and movement.
neuroretinitis - Inflammation of the optic nerve and retina, commonly caused by an infection. Symptoms include blurred vision, headache, floaters, eye pain or discomfort, vision loss and loss of color vision.
nevus - Birthmark, freckle or mole that is often brownish, but can be other colors as well. A nevus can occur on the skin or inside the eye and can become a melanoma, a type of cancerous growth. If your eye doctor discovers a nevus within your eye, he or she will want to check it regularly to see if it grows or becomes a melanoma that requires treatment.
nickel - Metallic element used mainly in alloys. Many eyeglass frames are made of nickel alloy, so people who are allergic should choose a hypoallergenic substitute, such as titanium.
nose pad - One of a pair of pads, usually clear, that rest on either side of your nose and help to support your glasses.
nystagmus - Rapid and involuntary eye movement that is oscillating and non-chaotic. Blurred vision may result. Nystagmus typically affects infants and has a variety of causes. Please read our article on nystagmus.
OD - Abbreviation for "oculus dexter," the Latin term for "right eye." Or, doctor of optometry.
ocular herpes - Recurrent viral infection that can cause inflammation and scarring of the cornea. It is not sexually transmitted. There are various types of ocular herpes, ranging from herpes keratitis to more serious forms that can lead to blindness. See our eye herpes article for more details.
ocular hypertension - Condition in which the intraocular pressure of the eye is elevated above normal and which may lead to glaucoma. More about high eye pressure.
ocular migraine - Visual phenomena that may accompany a migraine headache or that may occur without any headache. They include light flashes, spots, wavy lines, flickers, zig-zagging lights, semi-circular or crescent-shaped visual defects and distortions of shapes. See our ocular migraine article for more details.
onchocerciasis - Commonly called "river blindness," onchocerciasis is caused by a parasitic worm, which is spread in the human bloodstream through bites from blackflies and buffalo gnats found in parts of Africa, South America, and Central America. The worm's offspring cause inflammation, bleeding, and other problems in the eye. Without a 15-year regimen of annual doses of Mectizan, blindness will result. (Information supplied by ORBIS International.)
ophthalmologist - A medical doctor (MD) who specializes in the eye. Ophthalmologists perform eye exams, treat disease, prescribe medication, and perform surgery. They may also write prescriptions for eyeglasses and contact lenses.
ophthalmoplegia - Eye muscle paralysis. Causes include stroke, multiple sclerosis, a tumor, thyroid disease, migraines and progressive supranuclear palsies. Symptoms can include limited eye movement, blurred vision, double vision, nystagmus and ptosis. Tolosa-Hunt Syndrome, often called "painful ophthalmoplegia," is characterized by intense pain behind the eye and a headache.
opsoclonus - Rapid and involuntary eye movement that is irregular and chaotic; sometimes called "dancing eyes." Typically seen in Opsoclonus-Myoclonus Syndrome.
optic - The rounded, central portion of an intraocular lens (IOL) used in cataract surgery. Optic also more generally refers to eyes or vision.
optical coherence tomography - A method of imaging that, in ophthalmology, uses light waves to provide cross-sectional views of interior eye structures. Also known as OCT.
optic nerve head - Also called optic disk. Circular area where the optic nerve enters the retina, and the location of the eye's blind spot.
optic nerve problem - The optic nerve (second cranial nerve) is the part of the eye that carries stimuli from the rods and cones to the brain. Problems such as inflammation (optic neuritis), tumors and swelling can lead to symptoms such as: blurred vision, loss of color vision, floaters, headache, eye pain or discomfort, nausea and vision loss.
optician - In the United States, opticians are not doctors, but in some states they must complete training and be licensed. And in some states they can, after special training, become certified to fit contact lenses. (Please visit the Opticians Association of America website for licensing requirements for various states.) Most opticians sell and fit eyeglasses, sunglasses, and specialty eyewear that are made to an optometrist's or ophthalmologist's prescription. Many also have equipment on the premises so they can grind lenses and put them in frames without ordering from a lab.
optometrist Doctors of optometry (ODs) examine eyes for both vision and health problems, prescribe glasses, and fit contact lenses. They can prescribe many ophthalmic medications and may participate in your pre- and postoperative care if you have eye surgery. ODs must complete four years of post-graduate optometry school for their doctorate.
orbit - Eye socket. Read more about the orbital bone.
orbital cellulitis - A sudden infection of the tissues immediately surrounding the eye, resulting in painful swelling of the upper and lower eyelid, and possibly the eyebrow and cheek. Other symptoms include bulging eyes, decreased vision, fever, and eye pain when moving the eyes. Bacteria from a sinus infection are a common cause; other causes include a stye on the eyelid, bug bites or a recent eyelid injury. Orbital cellulitis is a medical emergency and prompt IV antibiotic treatment often is needed to prevent optic nerve damage, permanent vision loss or blindness and other serious complications.
orbital pseudotumor - An inflammatory mass in the tissues around or behind the eye that looks like and mimics the symptoms of a tumor. The cause is unknown. The primary symptom is a painful, bulging eye. You may also experience pain or discomfort around the eye.
orphan drug - A drug designated for treatment of a rare condition or disease that typically affects fewer than 200,000 U.S. residents. When the FDA grants orphan drug status, a company may be qualified for special grants, tax breaks or other incentives to help defray research and production costs. Without these incentives, companies would be unable to profit from development of orphan drugs because of limited demand.
orthokeratology - (ortho-k) Procedure in which a doctor fits you with special gas permeable contact lenses to reshape your cornea and correct errors like nearsightedness. Often, patients wear the lenses just at night. Learn more about ortho-k.
OS - Abbreviation for "oculus sinister," the Latin term for "left eye."
osteopetrosis - Rare, hereditary disease in which the bones are too dense. Common symptoms include bone pain and fractures. Retinal degeneration may occur; it results in vision loss.
OU - Abbreviation for "oculus uterque," the Latin term for "each eye," used in vision correction prescriptions to indicate both eyes. Also an abbreviation for "oculi unitas" or "oculi uniter," meaning both eyes working simultaneously together.
overconvergence - Condition in which the eyes come too far inward when focusing on a near object, resulting in blurring.
overflow tearing - Common congenital condition in infants and developmental condition in older adults caused by a blocked tear duct. In infants, a membrane blocks the tear drainage duct, resulting in excessive tears and mucus. In adults, the cause of the blockage is usually unknown, but can be related to poor eyelid function.
Page published in April 2019
Page updated in November 2021