When you and someone else aren't in a relationship such as boyfriend or girlfriend, but you have sex together. Can be an open or closed relationship.
Another name for friends with benefits
Person A: Are you guys boyfriend and girlfriend?
Person B: Nope we're in a fucklationship.
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.