n. a visual disorder in which the light rays of a visual stimulus do not all focus at a single point on the retina due to uneven curvature of the cornea or lens. As a result, light rays are refracted more in one meridian (that of the greatest curvature) than in the other at right angles to it (that of least curvature). The effect is an aberration or distortion of the visual image that makes it difficult to see fine detail. —astigmaticadj.