noun

• (sciences) A coherent statement or set of ideas that explains observed facts or phenomena and correctly predicts new facts or phenomena not previously observed, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc.
  "In its most proper acceptation, theory means the completed result of philosophical induction from experience."
• (uncountable) 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."
• (mathematics) A field of study attempting to exhaustively describe a particular class of constructs.
  "Knot theory classifies the mappings of a circle into 3-space."
• (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).
  "A theory is consistent if it has a model."