A supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.

The theory of relativity revolutionized our understanding of space and time.

A set of principles on which the practice of an activity is based.

In music theory, students learn about the structure and elements of music.

An idea used to account for a situation or justify a course of action.

Her theory was that the missing keys were somewhere in the house.

theoryadjective

Relating to theoretical knowledge or principles rather than practical application (used attributively).

All first-year students must take a theory course before the laboratory sessions.