1. Usually refers to members of the F.B.I., C.I.A., N.S.A., or any members of other covert, government run, or para-military organization or the organizations themselves. These organizations are sometimes involved with extraterestrial or other worldly matters. Members of these organizations usually wear black suits, drive black cars or S.U.V.'s, provide little, if any, information to members of the general public and act in a mostly unpredictable and paranoid manner.
2. The government it self(Usually used in a conspiritorial sense)
3. Any covert group
4. A popular 1997 science fiction/action film featuring Will Smith and Tommy Lee Jones
A popular quote from the character Zed(Rip Torn) from the above film describes the ways of The Men in Black and their cohorts:
Zed: You'll dress only in attire specially sanctioned by MiB special services. You'll conform to the identity we give you, eat where we tell you, live where we tell you. From now on you'll have no identifying marks of any kind. You'll not stand out in any way. Your entire image is crafted to leave no lasting memory with anyone you encounter. You're a rumor, recognizable only as deja vu and dismissed just as quickly. You don't exist; you were never even born. Anonymity is your name. Silence your native tongue. You're no longer part of the System. You're above the System. Over it. Beyond it. We're "them." We're "they." We are the Men in Black.
MiB's: an abbreviation for Men In Black
One day, the Men in Black showed up at my friend's house, and after that day, I never saw him again.