In Swinging, full swap is sex with another couple where both couples swap partners, such as the guy from one couple with the girl from the other couple and vice-versa. This is often done in the same room but applies also when separate rooms are involved.
Swinging Couple 1: Do you full swap or just soft swap?
Swinging Couple 2: We love watching each other have sex with someone else, so we usually full swap.
Swinging Couple 1: Sweeeeeet.