sleeper

• sleep‧er [countable]
• 1 someone who sleeps in a particular way
• light sleeper (=someone who wakes easily)
• heavy sleeper (=someone who does not wake easily)
• 2 someone who is asleep
• 3 British English a heavy piece of wood orconcrete that supports a railway track [= tieAmerican English]
• 4
• a) a night train with carriages that have beds for passengers to sleep in
• b) a sleeping car
• c) American English a bed on a train for a passenger to sleep in
• 5 especially American English a film, book etc which is successful, even though people did not expect it to be
• 6 American English a piece of clothing for a baby that covers its whole body including its feet
• 7 a spy who is sent to a particular place and who lives an ordinary life there until a later time, when they begin their spying activities

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.