The strong-CP problem is a particular inconsistency between experimental observations of strong interactions and the theory that describes them. Unlike in weak interactions, CP violation has never been observed in strong interactions. However, the theory that describes strong interactions allows CP violation to occur. So why is it not observed? That is the strong-CP problem. Various attempts to resolve the strong-CP problem have been proposed, but none have been experimentally verified. See: axion, charge, CP violation, parity.