A V10 is an engine in V configuration, having 10 cylinders.
The V10 configuration is inherently imbalanced in the vertical plane and generates a rocking couple causing vibration from end to end of the engine. To contain this issue a balance shaft is required. This makes the V10 difficult to engineer without CAD. Thus the V10 was never used before the 1980s.
V10s have rarely been a popular configuration for road cars: a V12 is almost as cheap to build and runs more smoothly. However, recent years have seen increasing use of the configuration in the United States. The Dodge Viper uses a V10 engine, and one is available on Ford F-250 and Ford F-350 trucks and the Ford Excursion large SUV.