theory

noun8

1. •
   a set of ideas, supported by facts and tests, that explains how or why something happens

   • In its most proper acceptation, theory means the completed result of philosophical induction from experience. — 1843
2. •
   A description of an event or system that is considered to be accurate.

   • As they encrease the hatred of vice in some, so doe they enlarge the theory of wickednesse in all. — 1646
3. •
   A hypothesis or conjecture.
4. •
   The underlying principles or methods of a given technical skill, art etc., as opposed to its practice.

   • Lopukhov wrote a number of books and articles on ballet theory, as well as his memoirs. — 1998
5. •
   The standardization and study of fixed sequences of moves, especially in the opening phase of a game.
6. •
   A field of study attempting to exhaustively describe a particular class of constructs.

   • Knot theory classifies the mappings of a circle into 3-space.
7. •
   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).

   • A theory is consistent if it has a model.
8. •
   (no longer used) Mental conception; reflection, consideration.