CPT symmetry is a fundamental symmetry in physics that applies to all physical phenomena. It stands for Charge, Parity, and Time reversal symmetry. The CPT theorem states that every relativistic quantum field theory has a symmetry that simultaneously reverses charge, reverses the orientation of space (or ‘parity,’ P), and reverses the direction of time (T) . CPT is the only combination of C, P, and T that is observed to be an exact symmetry of nature at the fundamental level. The implication of CPT symmetry is that a "mirror-image" of our universe would evolve under exactly our physical laws.

Experimental evidence is consistent with zero dipole moment, so time reversal symmetry seems to hold in most cases. However, there are some cases where CPT symmetry is violated, such as in the decay of the long-lived neutral K meson.

In summary, CPT symmetry is a fundamental symmetry in physics that applies to all physical phenomena. It stands for Charge, Parity, and Time reversal symmetry, and the CPT theorem states that every relativistic quantum field theory has a symmetry that simultaneously reverses charge, reverses the orientation of space, and reverses the direction of time. While CPT symmetry is observed to be an exact symmetry of nature at the fundamental level, there are some cases where it is violated.