A threesome is the term which describes sexual activity involving three people. The level of sexual activity among the three members may vary greatly. Some threesomes may involve one who has a voyeur role, sometimes the invited member is there for enhancing the experience which does not involve having intercourse with the couple role (this is sometimes referred to as soft-swinging), other threesomes may involve some same sex contact, and other threesomes actively involve all three members.

Sometimes the term threesome is confused with the term ménage à trois. Granted, both activities involve sex amongst three people. A threesome generally involves a couple that seeks out a third person for sex without the emotional entanglements that are normally seen in an open relationship. On the other hand, the term ménage à troisis implies a special form of a threesome with involves an emotional bond among all three people and in many ways resembles more of an open relationship than a threesome.