Abbreviation used by epidemiologists and others to designate "Men who have Sex with Men."
Epidemiologists, anthropologists, and other social scientists have long understood, as does The Man2Man Alliance, that the labels "gay," "bi," and "straight," are effectively meaningless, and that what matters in the real world is behavior.
Because all sorts of men have sex with men, not just men who self-identify as "gay" or "bi."
See also MLM for Men who Love Men.