The butt, behind, bum, backside, or ass was the part of the body on which a lifeform sat upon, situated at the end of their back and, in bipeds, above the legs, and in quadrupeds, above the rear legs.
Captain Jonathan Archer noted that he didn't recruit his tactical officer to sit on his butt when he's threatened, replying to Trip Tucker's comment that Malcolm Reed wouldn't be that reckless to risk the lives of the landing party following a threat by the Andorian Commander Thy'lek Shran in 2151. (ENT: "The Andorian Incident")
When The Doctor was testing his new program that made him able to daydream, he imagined Tuvok becoming destructive due to pon farr and himself singing to Tom Paris, instructing him to load a hypospray and "aim for [Tuvok's] behind" when given a signal by the doctor. (VOY: "Tinker Tenor Doctor Spy")
When Melora Pazlar was having medical treatment to make her adjust to the higher levels of gravity than she was used to, it increased the activity in her motor cortex, resulting in her neuromuscular tissue being stimulated and a sensation of her buttocks warming up. (DS9: "Melora")
When Zek was visiting Deep Space 9 one time, he pinched Kira Nerys on her behind. She tried to threaten to stick the brizeen nitrate he was planning to sell to Bajor up his behind lest he do it again, but was interrupted by Quark. (DS9: "Rules of Acquisition")