Final answer:
Astigmatism is a common vision defect caused by an unequal curvature of the cornea, resulting in uneven focusing and blurred images. It can be corrected with cylindrical lens corrections that counteract the irregular shape.
Step-by-step explanation:
An error in refraction caused by an unequal curvature of the cornea, which leads to unequal focusing and blurred images, is referred to as astigmatism. This condition is characterized by an unevenness or asymmetry in the focus of the eye. For example, rays passing through different regions of the eye may focus at varying distances, causing the image to appear elongated or distorted. Astigmatism is commonly due to irregularities in the shape of the cornea, but can also result from lens irregularities or unevenness in the retina. When correcting astigmatism, a cylindrical correction is used in eyeglasses, contrasting with the spherical corrections that are applied for short- or farsightedness, which are uniform along all axes.