1) Unequal curvature of the refractive surfaces of the eye. Thus a point source of light cannot be brought to a point focus on the retina but is spread over a more or less diffuse area. This results from the radius of curvature in one plane being longer or shorter than the radius at right angles to it. (Dorland, 27th ed)

2) Optical defect in which refractive power is not uniform in all directions (meridians); light rays entering the eye are bent unequally by different meridians, which prevents formation of a sharp image focus on the retina.