A gang bang is a situation in which several people, usually more than three, engage in physical sexual activity with one specific person sequentially or at the same time.
That specific person is the central focus of the sexual activity; for example, it could be one woman surrounded by several men, or a man with multiple women (a "reverse" gang bang).
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 or triple penetration.