A gang bang or gangbang is a situation in which several people engage in physical sexual activity with one particular individual sequentially or at the same time. Normally, there is a single individual who is the central focus of the sexual activity; for example, one man or woman surrounded by several men or women.
Rather than serial couplings by two people, the gang bang is defined by the number and simultaneity of the various sex acts, such as vaginal, anal or oral sex or double penetration.

Gang bangs are not defined by the precise number of participants, but usually involve more than three people and may involve a dozen or more.

Gang bangs also differ from group sex, such as threesomes and foursomes, in that most (if not all) sexual acts during a gang bang are centered on or performed with just the central person.

[From Wikipedia]