(sociology) Any of various models of group behavior (such as economic behavior, criminal behavior, or consumer behavior) as the cumulative result of rational choices made by individuals expressing on their preferences and/or meeting their needs.

Used other than figuratively or idiomatically: see group, theory; a theory about (particular kinds of) groups.

(mathematics) The mathematical theory of sets. / (music) Musical set theory, a systematic approach to describing musical objects and their relationships.

(mathematics) The mathematical study of probability (the likelihood of occurrence of random events in order to predict the behavior of defined systems).

(uncountable, mathematics) The branch of mathematics dealing with the properties of graphs (networks of vertices and edges).

(mathematics) A branch of topology dealing with knots.

(mathematics) The study of iterative non-linear systems in which arbitrarily small variations in initial conditions become magnified over time.

(mathematics) The branch of pure mathematics concerned with the properties of integers.

The informal theory that stock market prices move in the same direction as the hemline of women's skirts, short skirts being considered bullish and long skirts bearish.

Any theory attempting to explain the process by which individuals explain the causes of behaviour and events.