What is Theory ?
Theory is (noun) 1. an explanation of something which has not been proved but which you believe is true I have a theory which explains why the police never found the murder weapon. 2. a careful scientific explanation of why something happens Galileo put forward the theory that the earth turns round the sun. the theory of evolution a theory, developed by Charles Darwin, that species evolve by a process of natural selection 3. a statement of general principles which may not apply in practice In theory the treatment should work, but no one has ever tried it. (NOTE: The plural is theories.)
source: Easier English, Student Dictionary Upper Intermediate Level