Meaning of THEORY in English
■ noun ( plural theories )
1》 a supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.
↘an idea accounting for or justifying something.
2》 a set of principles on which an activity is based: a ~ of education.
↘ Mathematics a collection of propositions illustrating the principles of a subject.
in ~ in an ideal or hypothetical situation.
C16: via late L. from Gk theōria 'contemplation, speculation'.
Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004