An abbreviation, particularly useful and common in AIDS
prevention circles, to designate "M
en who have S
ex with M
en." It's a decidedly unwieldy phrase which makes the abbreviation MSM all the more handy.
It may not be obvious right away what the difference is between being a "gay" man and an MSM. In fact, many MSM's do consider themselves gay, either secretly, or not. However, there are a good number of men who routinely have sexual interaction with other males, and yet, for whatever reason, do not choose to don the "gay" label.
Health workers attempting to prevent the spread of AIDS were missing a large part of their target audience by seeking only those men who were "gay". As they later came to realize, the word "gay" carries a lot of connotations, implications, and baggage that many MSM's reject, like, for example, participation in a "gay community."
Using the term "MSM" attempts to cut through all of the negative connotations of the words "gay" and "bisexual", and simply addresses -- with no moral or social judgment passed in either direction -- the only relevant fact (for healthcare and social workers): does this person engage in relatively risky activities, or not?