Time reversal is the mathematical operation analogous to running a movie film backward, i.e. replacing time t with time -t. In other words, a transformation from agiven physical system undergoing a given sequence of events to a system in which the exact reverse sequence of states is undergone. The symmetry of time reversal (symbol T) is known as T invariance. As with CP violation, T violation occurs in certain weak interactions, notably kaon (see meson) decay. See also CPT theorem.