The CPT theorem is a basic principle in particle physics which states that the combined operation of charge conjugation (C), time reversal (T), and parity (P) in any order is an exact symmetry of any interaction and that under the three transformations all physical laws must be invariant. This means that if any particle is replaced with its corresponding antiparticle, and the space coordinate and time are reversed, the physical laws are unchanged.
According to the mathematics of the theorem, CPT may be invariant if each of the independent parameters C, P, and T is invariant. If one of the parameters is not conserved then there must also be some compensating violation in one of the other parameters in order for the theorem to be sound. See also CP violation.