1. In optics, a specific deviation from a perfect image, such as spherical aberration, astigmatism, coma, curvature of field, and distortion. For example, spherical aberration results in a point of light (that is, a point image) appearing as a circular disk at the focal point of a spherical LENS or curved MIRROR. This occurs because the focal points of light rays far from the optical axis are different from the focal points of light rays passing through or near the center of the lens or mirror. Light rays passing near the center of a spherical lens or curved mirror have longer focal lengths than do those passing near the edges of the optical system.
2. In astronomy, the apparent angular displacement of the position of a celestial body in the direction of motion of the observer. This effect is caused by the combination of the velocity of the observer and the velocity of light.
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