Astigmatism: A common form of visual impairment in which part of an image is blurred due to an irregularity in the dome-shaped curvature of the front surface of the eye, the cornea. With astigmatism, light rays entering the eye are not uniformly focused on the retina. The result is blurred vision at all distances. Significant astigmatism can cause headaches, eye strain, and seriously blurred vision. Astigmatism is often not detected during routine eye screening in schools. It may coexist with other refractive errors such as nearsightedness and farsightedness. Astigmatism is corrected with slightly cylindrical lenses that have greater light-bending power in one direction than the other. Use of these lenses elongates objects in one direction and shortens them in the other, much like looking into a distorting wavy mirror.