1. a person or animal who is asleep or who sleeps in a specified way: "he was a light sleeper, for long periods an insomniac"
2. a train carrying sleeping cars: "a sleeper train"
▪ a sleeping car, or a berth in one.
▪ one-piece coverall pajamas for a baby or small child.North American
▪ a sofa or chair that converts into a bed.North American
3. a movie, book, play, etc., that achieves sudden unexpected success after initially attracting little attention, typically one that proves popular without much promotion or expenditure: "this low-budget, black-and-white American sleeper concerns two brothers"
▪ an antique whose true value goes unrecognized for some time: "photographed after it proved to be a sleeper, this Regency table took $8,600 instead of $1,500 as anticipated"
▪ a secret agent who remains inactive for a long period while establishing a secure position: "a KGB plot to uncover his sleepers"
4. a railroad tie.British
5. a stocky fish with mottled coloration which occurs widely in warm seas and fresh water.