An FMF (or female-male-female) threesome is a threesome that involves two females and a male. The letters FMF are placed in this order deliberately; in threesome lingo, the letter in the middle - in this case, it refers to the male participant - is supposed to be the focus of the threesome. To many this means that in an FMF threesome, the female participants do not interact sexually with each other. This would, however, happen in an FFM threesome.
Some people see FMF threesomes as being a purely male fantasy, and not about female pleasure. However, this really depends on the values of the people involved, and how the threesome is negotiated among them. FMF threesomes can include a range of different activities that are up to the participants involved. FMF threesomes are common in pornography.
A single person (most often a woman) who joins a couple for a threesome is often called a "unicorn" because this type of person is considered hard to find.
