A sleeper ship was a late 20th to early 21st century Earth space vessel in which the crew could be placed into suspended animation for all or part of the journey. At least some DY-100-class ships were sleeper ships. As Marla McGivers put it in 2267, sleeper ships were "necessary because of the time involved in space travel until about the year 2018. It took years just to travel from one planet to another." (TOS: "Space Seed")
Despite this, around 2210, Harry Kim's "uncle" Jack piloted a sleeper ship on a deep-space exploration trip to Beta Capricus. The rest of the crew went into stasis while Jack spent the six months piloting the ship himself alone. Upon arrival, the "star" was found not to exist and to be only an electromagnetic echo of a distant galaxy. Jack chose not to wake the rest of the crew before returning, after a further six-month journey back to Earth. When they awoke in Earth orbit they initially questioned why they had never left. (VOY: "11:59")
Background information Edit
The term "sleeper ship" was only ever explicitly applied to the SS Botany Bay, although it is possible that DY-500-class ships could also be called this. Other ships that utilized stasis chambers (a technology apparently broadly synonymous with "suspended animation" in VOY: "One"), such as the Klingon ship the IKS T'Ong in TNG: "The Emissary", were not referred to by that term. This term should not be confused with "generational ships," wherein the original crew lives out their lives with subsequent generations taking over after their deaths.