astigmatism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A visual defect in which the unequal curvature of one or more refractive surfaces of the eye, usually the cornea, prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision.

from The Century Dictionary.

• noun In ophthalmology, a defect in the refractive apparatus of the eye, the curvature of the refracting surfaces being greater along certain meridians than along others, so that rays of light proceeding from an external point do not converge to a point upon the retina, but to a line.
• noun A similar defect in a lens.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Med. & Opt.) A defect of the eye or of a lens, in consequence of which the rays derived from one point are not brought to a single focal point, thus causing imperfect images or indistinctness of vision.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun optics A defect of a lens such that light rays coming from a point do not meet at a focal point so that the image is blurred.
• noun pathology A disorder of the vision, usually due to a misshapen cornea, such that light does not focus correctly on the retina causing a blurred image.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (optics) defect in an optical system in which light rays from a single point fail to converge in a single focal point
• noun (ophthalmology) impaired eyesight resulting usually from irregular conformation of the cornea; common in nearsighted people