In physics, particles have a quantum mechanical property of parity. The corresponding physical quantity is either equal to +1 or -1 and the parity of a composite system is the product of the parities of the individual subsystems: parity is a multiplicative quantity. According to quantum mechanics, the conservation of this parity is equivalent to the laws of physics being invariant under mirror reflection. It has been found that parity is not conserved in the weak force and so the weak force is not invariant under mirror reflection.
See: Parity transformation, P-symmetry, intrinsic parity.