Where three (3) people engage in a sex, where the third is a stranger.
Where three (3) people engage in a sexual tryst, where two of the participants are involved with each other, the third being a stranger. In addition, one of the involved partners is merely a spectator to the sexual activity, and not an active participant.
Unlike a Menage a Trois, this relationship does not require that there be a romantic interest between all three parties.
This term was coined in 1941, and listed as a 'Paraphilia'.
Basically a couple, engage a third party, for sexual activity.
There are instances where the third party is actually the one doing the watching, though not common.
For one of the related partners, to be hidden, is also a common occurrence, though not a strict requirement.
Simply three people, where one is the watcher of the other two, and where only two of the participants are related or involved romantically, together.