Any individual who intrudes or does not belong- in a conversation, at a social gathering, or anywhere else. First popularized by Malachai, of "Children of the Corn".
Friend 1: "Yo, who invited him? Who's that guy lurking in the corner?"
Friend 2: "I don't know who that interloper is, dude."
The friend that moves in on the guy who's talking to you at the bar. She thinks she is the cutest of the group, and therefores feels entitled to any man, attractive or not, particularly if he's talking to someone else.
That Nicole is such an interloper. She's talking to the guy that Krista likes while she's in the bathroom.