a person or thing that boosts, especially an energetic and enthusiastic supporter.
2.
Electricity. a device connected in series with a current for increasing or decreasing the nominal circuit voltage.
3.
Railroads. any machine, device, phenomenon, etc., that helps to move a train, as a tailwind, downgrade, roller bearings, or especially a helper locomotive.
4.
Military. an explosive more powerful than a primer, for ensuring the detonation of the main charge of a shell.
5.
Rocketry.
a.
a rocket engine used as the principal source of thrust in the takeoff of a rocket or missile.
b.
the first stage containing this engine and its fuel supply, which may or may not be detached from the rocket when the fuel has been consumed.