When you and your partner (i.e. same sex, opposite sex) sleep around either together or apart. Both parties are fully aware of the other person's activities.
When you decide to also bring a third party into the relationship and constantly refer to yourselves as "the power of 3" (i.e. Charmed -- cause there's nothing "charming" about that).
Guy #1: Wait, I didn't know that they were ALL together in a relationship? What is that? I mean... how??
Guy #2: That's not a relationship. That's a fucklationship.