Definition of “theory” - Learner’s Dictionary

theory

noun uk us /ˈθɪəri/
IDEA [ C ]

B2 an idea or set of ideas that is intended to explain something:

Darwin's theory of evolution
PRINCIPLES [ U ]

the set of principles on which a subject is based:

economic theory
in theory

B2 If something is possible in theory, it should be possible but often it does not happen this way:

In theory, the journey should be shorter but in practice, the roadworks slow you down.

(Definition of “theory” from the Cambridge Learner's Dictionary © Cambridge University Press)