Evolving wrote:(sorry to be a geek, but:)

The theory states that the black hole in an active galactic nucleus has to have around 10^8 solar masses; whereas other black holes (such as those that we think we have detected in X-ray active binary systems) are much smaller: say 10 or so.

Hence "supermassive".

(There isn't one of those at the centre of our galaxy, however (fortunately for us), so the term is in fact not appropriate here.)

(I have got a sense of humour, honest...)

The most recent estimates of Sgr A*'s mass is 4.3 +_ 0.4 million solar masses with the main uncertainty deriving from the distance between the object and the Sun. I'm not sure there is any strict definition of supermassive black hole as I have seen the term used for any black holes significantly more massive than the most massive stars. Stefan Gillessen and co, authored this paper showing how they calculated Sgr A*'s mass based on observations of the stars in close orbits around it.

http://arxiv.org/abs/0810.4674

In about five years, the distance uncertainty will be greatly reduced if the ESA's Gaia mission is successful.