The relationship between two people who are not dating and have no stated boundaries, but are more to each other than just a booty-call.
Mara: "Chelsea, are you and DB still hooking up with each other or have you two finally decided to make it official"
Chelsea: "No way...we will never be official! We are just two friends who enjoy each other both with and without our clothes on."
Mara: "Oh word ... so you two are just "Less Than Casual"!