A sleeper (US English) or Q-car (British English) is a car that has high performance and an unassuming exterior. Sleeper cars are termed such because their exterior looks little or no different from a standard or economy-class car (or in some cases worse due to seeming neglect on the owner's part), but internally they are modified to perform at higher performance levels. The American term possibly comes from how an aggressive animal can seem gentle or perhaps friendly until awoken, while the British term derives from the Q-ships used by the Royal Navy.

The earliest known reference to the term 'Q-car' is in the February 1963 edition of Motor Sport magazine (published in the UK since 1924). The editor, Billy Boddy, said of the Lotus Cortina ~ "...the modifications carried out by Lotus have turned it in to a 'Q' car par excellence...". Some cars are equipped this way at the factory to suit the tastes of those who want performance without attracting attention of the police or car thieves.

For instance, many high-performance sedans look hardly any different from the lower-powered models in the range. Cars with obvious external badging, or overt visual elements that give the impression of high performance, are not true sleeper cars.

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