theory

An idea about why something happened or what is true, which has not been proved.

• I have a theory about the noise.
• Her theory was that the dog had escaped.
• The police considered several theories about his disappearance.

How to use it: a theory about something (Do you have a theory about the missing money?); a theory that… (My theory is that someone left the door open.)

Similar words: idea, guess, hypothesis, conjecture

Common phrases: my theory, one theory, theory about something

A set of ideas that explains how or why something happens, usually supported by evidence.

• This theory explains how stars form.
• New evidence supports the scientist's theory.
• Researchers tested several theories about the disease.

How to use it: a theory of something (We discussed the theory of evolution in class.)

Common phrases: scientific theory, theory of evolution, test a theory

A subject that studies the rules and ideas behind a particular kind of thing.

• She studies number theory.
• Our music theory course starts next week.
• Graph theory helps researchers understand networks.

Common phrases: number theory, knot theory, music theory, set theory