A black hole binary (or BHB or binary black hole or BBH) is two black holes co-orbiting. They could be the remnants of binary stars, or could be two supermassive black holes from a galaxy merger (a binary SMBH). A source of detectable gravitational waves is a binary black hole whose orbit has decayed to the point that the two merge (a black hole merger).
The term black hole binary (or BHB) is also used differently: to specifically mean a binary system in which at least one of the objects is a black hole, the other of which might be something else (main sequence star, white dwarf, neutron star, etc.).