Astigmatism is an optical defect or refractive error of the optical system of the eye that results in blurred vision for both distance and near viewing. In this refractive state, a point object is imaged as two lines that are perpendicular to each other and at a different focal distance from the lens. This property has given rise to the name "astigmatism" (a = not; stigma = point). The most common reason for this refractive state is that the cornea, which is the major refracting surface of the eye, is not spherical but is warped along a certain meridian. Allowing for the effect of the lens in the eye, the two line images of a point object correspond to the flattest and steepest corneal meridians. The difference in curvature between these meridians determines the degree of astigmatism of the cornea.

If the flattest meridian of the cornea is closest to the horizontal meridian, the cornea is said to have "with-the-rule" (WTR) astigmatism. If it is closest to the vertical meridian, the cornea is said to have "against-the-rule" (ATR) astigmatism. To a lesser extent, the shape of the lens within the eye can also contribute to the eye's overall astigmatism. After two years of age, the normal eye (no overall astigmatism) has a small amount of WTR astigmatism of the cornea that is corrected by an equal and opposite (ATR) astigmatism of the lens in the eye. For a diagrammatic explanation of the optics of astigmatism please refer to OAA description of astigmatism.

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