1. A defect in a lens of an eye that prevents light rays from meeting at a single point, thus producing an imperfect image.
2. A common form of visual impairment in which part of an image is blurred, due to an irregularity in the curvature of the front surface of the eye, the cornea.

The curve of the cornea is shaped more like an American football or a rugby ball rather than normally as a spherical basketball.

Light rays entering the eye are not uniformly focused on the retina; resulting in blurred vision at all distances and only part of what a person is looking at is in clear focus at any one time.

3. Etymology: coined in form of astigmatic in 1849 by English scientist William Whewell (1794-1866), from Greek a-, "without" + stigmatos, stigma, "a mark, a spot, a puncture".

A physical condition that is caused by a defective curvature or a refractive surface of the lens of an eye: A linticular astigmatism is an abnormal situation in which a refraction or a change in direction of a ray of light in the eye is not sharply focused on the retina, but it is spread over a more or less diffuse or scattered area.