Adult buffet is a colloquial term to describe a gathering of adults who come together to have sex with one another. Each person attending an adult buffet is free to have sex with any other attendee, so long as all parties freely give consent. The term refers to the way people attending an adult buffet can pick and choose the people they want to engage with, much like the way people pick and choose the foods they want to enjoy at a traditional buffet.
The term originated within the swinging community. It is not commonly used outside this alternative lifestyle group.