|Owner:||United Federation of Planets|
The Endeavour was on a deep space exploration mission near Klingon territory in 2293. This ship, along with the USS Ahwahnee and the USS Emden, was selected as additional support for Operation Retrieve, which would have rescued Captain James Kirk and Doctor Leonard McCoy from Rura Penthe. (Star Trek VI: The Undiscovered Country)
This starship and its registry were not referenced in dialog. This vessel appeared in the plans for Operation Retrieve presented by Colonel West. Save for the USS Excelsior, the Endeavour and five other starships were graphically represented by the silhouette of a movie era refit Constitution-class vessel.
From this, the Star Trek Encyclopedia listed the ship as a Constitution-class vessel.
The Endeavour was probably named for the HMS Endeavour, specifically, the HM Bark Endeavour commanded by Lieutenant James Cook in his voyage of exploration in the Pacific from 1768–1771, and the NASA space shuttle Endeavour.
The Pocket TOS Errand of Vengeance novel Killing Blow features a USS Endeavor engaging Klingon forces at the Battle of Donatu V in 2245. This Endeavor was referred to as an Icarus-class at Memory Beta, the wiki for licensed Star Trek works starship in Errand of Fury: Seeds of Rage.