A junkyard, ship graveyard or surplus depot was a disposal site for defective, damaged, outmoded, and otherwise unused equipment.
After entering the Delta Triangle in 2269, Captain James T. Kirk of the USS Enterprise observed that what they discovered was "like a vast Sargasso Sea. A graveyard of ships from every civilization imaginable." (TAS: "The Time Trap")
The Qualor II Surplus Depot Z15 was a Federation junkyard that contained the hulks of numerous retired starships, along with at least two space stations. Among the identified vessels that were located at the depot were the Vulcan transport T'Pau, located at Section 18-Alpha-12, and the holding vessel USS Tripoli, located along the outer rim. (TNG: "Unification I", "Unification II")
When Miles O'Brien checked the remaining technological components of the SS Santa Maria on Orellius to contact the runabout he found nothing. Everything was removed and it looked like the ship went through a Ferengi junkyard. (DS9: "Paradise")