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1. An initialism for "mainstream media," referring to well-established media outlets, such as newspapers and TV stations, that reach a mass audience, considered collectively. The term is especially used to contrast it with other forms of media that one considers more truthful and/or less biased. It seems that people are becoming increasingly distrustful of the MSM.
2. An initialism for "men who have sex with men," referring to men who engage in such sexual activity but do not necessarily self-identify as homosexual or bisexual. In the medical community, use of the term "MSM" has been seen as a way to more accurately ascertain disease rates, by focusing on one's actual sexual activity, rather than their sexual identity.