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

(chemistry) An obsolete scientific theory of combustion.

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

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

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

(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.