Real Dictionary

retrorocket in American English
a small rocket on a larger rocket or spacecraft, that produces thrust in a direction opposite to the direction of flight in order to reduce speed, as for maneuvering
noun: a small, auxiliary rocket engine, forming a part of a larger rocket vehicle and having its exhaust nozzle pointed toward the direction of flight, for decelerating the larger rocket, separating one stage from another, etc

retrorocket in British English
noun: a small auxiliary rocket engine on a larger rocket, missile, or spacecraft, that produces thrust in the opposite direction to the direction of flight in order to decelerate the vehicle or make it move backwards