a person or thing that boosts, esp. an energetic and enthusiastic supporter.

Electricity . a device connected in series with a current for increasing or decreasing the nominal circuit voltage.

Railroads . any machine, device, phenomenon, etc., that helps to move a train, as a tailwind, downgrade, roller bearings, or esp. a helper locomotive.

Military . an explosive more powerful than a primer, for ensuring the detonation of the main charge of a shell.

Rocketry .

a rocket engine used as the principal source of thrust in the takeoff of a rocket or missile.

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.

Medicine/Medical . Also called booster dose, booster shot. a dose of an immunizing substance given to maintain or renew the effect of a previous one.

Pharmacology . a chemical compound, medicinal substance, or the like, that serves as a synergist.

a radio-frequency amplifier for connecting between a radio or television antenna and the receiving set to intensify the received signal.

an auxiliary pump, used in a pipeline or other system, to add to or maintain a prevailing amount of pressure or vacuum.

Slang . a shoplifter or petty thief.

noun

promoter , sponsor , patron , enthusiast , rooter , backer