(mathematics) A branch of mathematical analysis, concerned with the theory of integration, that generalizes the intuitive notions of length, area and volume.

(computer science) The study and classification of decision problems by the computational resources—such as time and space—needed by the programs that solve the problems.

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.

(biology) The scientific theory that all living organisms are made of cells as the smallest functional unit.

(originally law) Used other than figuratively or idiomatically: see conspiracy, theory. A hypothesis alleging that the members of a coordinated group are, and/or were, secretly working together to commit illegal or wrongful actions including attempting to hide the existence of the group and its activities.

(mathematics) A mathematical theory that models a cluster structure in a random graph to describe the spread of phenomena passing through the edges.

(mathematics, economics, dated) game theory.

(mathematics, economics) A branch of applied mathematics that studies strategic situations in which individuals or organisations choose various actions in an attempt to maximize their returns.

(physics) A generalized theory of eleven-dimensional supergravity that attempts to unify the five superstring theories

(mathematics) A branch of applied mathematics and engineering involving the quantification of information sent over a communication channel, disregarding the meaning of the sent messages, exemplified by the noisy-channel coding theorem.