astigmatism

n. unequal curvature of the lens of the eye creating a distorted image

In optics, astigmatism (from Greek: α- a- "without" + στίγματος stigmatos, gen. of στίγμα stigma "a mark, spot, puncture") is when an optical system has different foci for rays that propagate in two perpendicular planes. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances.
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Astigmatism is the result of an inability of the cornea to properly focus an image onto the retina. [more]Astigmatism - Community and Resources

Noun
1. (ophthalmology) impaired eyesight resulting usually from irregular conformation of the cornea
(synonym) astigmia
(hypernym) ametropia
(hyponym) keratectasia
(classification) ophthalmology
2. (optics) defect in an optical system in which light rays from a single point fail to converge in a single focal point
(synonym) astigmia
(antonym) stigmatism
(hypernym) condition, status
(classification) optics

(n.)
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.