Final answer:
Astigmatism is a vision condition caused by an irregularly shaped cornea or lens, leading to a distorted or elongated image, which is correctable with cylindrical lenses.
Step-by-step explanation:
The condition astigmatism is the result of irregularity of the lens or cornea of the eye which prevents focusing some parts of the image. This occurs when there are unevenness or asymmetries in the shape of the cornea, or sometimes, irregularities in the eye's lens or retina. As a result, rays of light passing through different regions of the eye may focus at different points, causing the image to appear distorted or elongated along certain axes. Astigmatism is generally noticeable as less distinct vision or images that are sharper along certain orientations.
To correct the vision problem caused by astigmatism, corrective lenses with a cylindrical correction are often prescribed. These lenses have an opposite irregularity to the eye's own to compensate for the distortion. By contrast, normal corrections for nearsightedness or farsightedness use spherical corrections that are uniform in all axes of the lens.