(chemistry) An obsolete scientific theory of combustion.

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

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.

(ecology) The theory that mitochondria and chloroplasts, which are organelles of eukaryotic cells, originated as independent organisms.

(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.

(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.

(mathematics, economics, dated) game theory.

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

An overall explanation of phenomena in a particular discipline or realm of experience such as economics, sociology, or history.