(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.

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

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

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

(mathematics, economics, dated) game theory.

(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) 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

(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.

(algebra, field theory) The branch of mathematics dealing with Galois groups, Galois fields, and polynomial equations.