A couch or sofa was a piece of furniture used for sitting on.
A Romulan Bird-of-Prey that played a prime factor in the Neutral Zone Incursion of 2266 had a couch on the spacecraft's bridge. At one point, the vessel's commander briefly sat on it. After a Romulan centurion was wounded in the same room, however, he lay on the couch, where he later died from his injuries. (TOS: "Balance of Terror")
Later, actress Lenore Karidian, who had played Lady Macbeth in the play, and Captain James T. Kirk, who had observed the performance, sat on a couch at a cocktail party on Planet Q. (TOS: "The Conscience of the King")
Captain Jean-Luc Picard's ready room aboard the USS Enterprise-D also featured a couch. (TNG: "Encounter at Farpoint", "Qpid", "Chain of Command, Part II" et al.) Another couch was owned by the O'Brien family on space station Deep Space 9, and yet another was owned by Ensign Harry Kim aboard the USS Voyager. (DS9: "The Darkness and the Light"; VOY: "Lineage")
After Tom Paris joked that – if Voyager went through a wormhole into the Alpha Quadrant – he and the rest of the starship's crew might end up in Harry Kim's parents' living room, Kathryn Janeway told Kim that he might have to ask his mother to move the sofa. (VOY: "Endgame")
Couches could also be used for sleep. Furel decided he and Lupaza would sleep on the O'Briens' couch to guard Kira Nerys, resolving that, though it was "a little short," the couch was at least as comfortable as their bed. (DS9: "The Darkness and the Light") Harry Kim offered Tom Paris the use of his couch to sleep on after the latter officer had quarelled with a pregnant B'Elanna Torres. (VOY: "Lineage")