CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. When it was found that both these symmetries were violated individually, it looked plausible that a combination of the two would be preserved by all physical laws. Simply stated, the preservation of CP-symmetry by all physical phenomena would mean that, all physical laws would preserve form when a charge-inversion transformation (positive to negative and vice-versa inversion of electric charges) and a parity-inversion transformation ('left' to 'right' and vice versa inversion, or, simply the reversal of the coordinate axes in a Cartesian coordinate system used to describe the system under consideration) are done simultaneously. But to the dismay of physicists in the 1950s, it was found that this symmetry too was violated and only a much stronger version of the symmetry could be preserved by physical phenomena, which was a CPT-symmetry.
