a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena: Einstein's theory of relativity.

a proposed explanation whose status is still conjectural and subject to experimentation, in contrast to well-established propositions that are regarded as reporting matters of actual fact.

Mathematics . a body of principles, theorems, or the like, belonging to one subject: number theory.

the branch of a science or art that deals with its principles or methods, as distinguished from its practice: music theory.

a particular conception or view of something to be done or of the method of doing it; a system of rules or principles: conflicting theories of how children best learn to read.