(black hole with a mass of millions to billions of solar masses)
A supermassive black hole (SMBH) is
a very large black hole, generally 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, sometimes cited as 108 through
109 as typical (as opposed to a stellar-mass black hole,
likely to be about 5 to 50 solar masses).
As of 2025, a number of SMBHs have been determined to be
at least 10 billion solar masses.
(Mass determinations take time to confirm or disprove, and acceptance
may be come gradually, so any determination of "the largest known"
has caveats.)
A SMBH has been detected and observed at the center of the
Milky Way (named Sag A*), and it is thought
that galaxies larger than dwarf galaxies generally host them.
Such large black holes are considered the energy source
for active galactic nuclei (AGNs), and galaxies without an AGN are presumed
to host a dormant SMBH.
Some methods of estimating the mass of SMBHs:
There is a mystery regarding SMBH formation
since the Eddington limit would seem to regulate
their growth. Current SMBHs have had enough time to grow, but
quasars at extreme redshifts suggest a power source
that only an SMBH can provide, which would have had insufficient
time to grow, and furthermore, there have not been observations
suggesting some kind of atypical rapid growth in process.
Regarding terms, SBH is occasionally used to mean SMBH,
but perhaps more often to mean "stellar black hole".
SMBH unfortunately might be used or read to mean
"stellar mass" or "solar mass", but its use as "supermassive"
appears pretty consistent.