In physics, a classical theory usually refers to a theory that does not obey the principles of quantum mechanics (classical theory vs. quantum theory). For example, see classical mechanics.

Less frequently, the term is used to label a non-relativistic theory - a theory that does not respect the rules of special relativity.

Quantum theory works at the microscopic levels but towards macroscopic limits it simplifies to classical theory while classical theory does not at all explain for quantum interactions.