The Enterprise on a routine training mission.
A training vessel is a starship that is used to train cadets to fly real starships. Typically, the training vessel's crew would go on training cruises.
The Federation Starfleet designated various vessels to be training vessels, including the refitted USS Enterprise and the USS Republic. (Star Trek II: The Wrath of Khan; DS9: "Valiant")
Starfleet also maintained a J-class vessel for training cruises until at least 2266, when Fleet Captain Christopher Pike suffered severe delta radiation exposure and eventual paralysis whilst attempting to rescue cadets trapped in the engine room. (TOS: "The Menagerie, Part I")