1. the mirrorlike correspondence of parts on opposite sides of a center, providing balance and harmony in the proportions of objects. This is considered an aesthetically pleasing quality. Distinct patterns of symmetry are widely produced. For example, almost all ornamental bands fall into one of seven types of band symmetry, and essentially all patterned wallpaper can be grouped into 17 types of plane symmetry.

2. one of the gestalt principles of organization. It states that people tend to perceive objects as coherent wholes organized around a center point; this is particularly evident when the objects involve unconnected regions bounded by borders. Also called law (or principle) of symmetry.

3. in mathematics and statistics, equality relative to some axis. More specifically, it is a condition in which values are arranged identically above and below the middle of a data set (see normal distribution) or above and below the diagonal of a matrix. Many standard statistical techniques are appropriate only for symmetrical data, such that nonsymmetrical data often are transformed into a roughly symmetrical form prior to analysis. Compare asymmetry.

4. see stimulus equivalence. —symmetrical adj.