The combination of Charge and Parity in physics. While a combined Charge, Parity, and Time (CPT) transformation leaves a system invariant some systems are not invariant under a combination of just CP transformations. This was first proposed by T.D. Lee and C. N. Yang in the 1950's and experimentally verified by C.S. Wu a few years later. Lee and Yang recieved the Nobel prize for this work.
A charge transformation is when you take the charge of a particle and swap it's sign

e ---} -e

A parity transformation is when you change the sign of the space coordinate of the particle

x ---} -x

A time transformation ,well go on guess.