Symbol T. The operation of replacing time t by time −t. The symmetry of time reversal is known as T invariance. As with CP violation, T violation occurs in certain weak interactions, notably kaon decay and processes involving mesons with charm and bottom quarks. It is also thought that violation of T should occur for neutrinos, because there are three types of neutrino, with these types having different masses; however, this has not been observed yet. See also CP invariance; CPT theorem.