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Sclera - The outer coating of the eyeball that forms the white of the eye; surrounds the optic nerve at the back of the eyeball.

Scleral contact lenses – Lenses that vault over the cornea and are supported by the white portion of the eye.

Slit lamp - Table-top microscope for examining the eye.

Snellen chart - An eye chart used to test a patient's vision.

Snellen line - A line of same-sized letters on an eye chart that is used to test a patient's vision.

Soft contact lenses - A contact lenses made of gel-like plastic containing varying amounts of water.

Soft Perm lenses - A soft lens with a hard center

Spot scanning laser - A medical instrument that produces a powerful beam of light that is focused at close range to remove corneal tissue. Spot scanning lasers use radar technology to track the eye's movement.

Stroma - Thickest part of the cornea (450-600 microns - approximately .5 millimeters). Between Bowman's membrane and Decemet's membrane.

Symmetry of refractive error - The refractive error in both eyes are close to the same value.

Tear film - A very thin film of water and other chemicals riding on top of the epithelium that lubricates the front of the eye.

Topical proparacain hydrochloride - Anaesthetic eye drops.

Toric - A specialist lens design that employs two different optical powers at right angles to each other for the correction of astigmatism.