A Supermassive Black Hole (SMBH) is a large Black Hole (BH), often in the center of a Galaxy. A black hole is considered "supermassive" if its mass is on the order of millions (or hundreds of thousands) of solar masses or more. The largest observed as of 12/2012 is on the order of 20 billion solar masses.
Such large black holes are considered the energy source for an Active Galactic Nucleus (AGN). Such a black hole has been detected in the center of the Milky Way by observing the tight orbits of stars around it.
A Supermassive Binary Black Hole (SMBBH) is two orbiting SMBHs. They are thought to result from Galaxy Mergers.
Active Galactic Nucleus (AGN)
Black Hole Binary (BHB)
Black Hole (BH)
Final Parsec Problem
Gravitational Wave (GW)
Nanohertz Gravitational Waves