No-hair conjecture (1960s): The conjecture (confirmed in the 1970s and 1980s) in general relativity that a black hole has merely three salient external characteristics: angular momentum, mass, and electric charge. All the other properties (comprising baryon number, lepton number, strangeness, and so forth) are destroyed as matter falls into the horizon.
Note that there is certain indication that quantum mechanical considerations in quantum gravity will outcome in a "quantum hair" coming into play. Though, that (a) would comprise a prediction of a theory that does not yet formally exist, and (b) is completely unimportant for solar-massed black holes, the only kinds that can be formed nowadays.