Definition of "theory"
theory
/ˈθɪə.ɹi/
noun
countable and uncountable, plural theories
(mathematics) A field of study attempting to exhaustively describe a particular class of constructs.
(countable, logic) A set of axioms together with all statements derivable from them; or, a set of statements which are deductively closed. Equivalently, a formal language plus a set of axioms (from which can then be derived theorems). The statements may be required to all be bound (i.e., to have no free variables).
(obsolete) Mental conception; reflection, consideration.