, light that enters the eye
is refracted more in one direction than the other, allowing only part of the object to be in focus at one time.
95% of the time, this is caused by differences in curvature in different axes of the cornea on the front of the eye. The cornea does 2/3 of the focusing of light in the eye. The other 1/3 is done by the lens, which causes 5% of astigmatism.