1. A defect in a lens or mirror that prevents light rays from meeting at a single point, 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 a normal spherical basketball.

Light rays entering the eye there are not uniformly focused on the retina. Rays entering through the more-curved surface are focused before the rays coming through the less-curved surface.

The light is focused clearly along one plane, but is blurred along the other. The result is blurred vision at all distances. Only part of what you are 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".

Astigmatism due to a defective curvature or a refractive surface of the lens of an eye.