Answer
Astigmatism is a defect where the lens of the eye is not spherical, i.e., it is more curved in one direction than another. It can be corrected with lenses whose curvature also varies with direction.
Work Step by Step
Visualizing the side of a barrel may help you picture an astigmatic lens, which is more curved in one direction than another. A section cut from a barrel side has a small, sharp radius of curvature around a vertical axis, and a more gentle curvature around a horizontal axis.
This is discussed on page 537.
