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")
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.