n. 1. (Elec.) An instrument for regulating the electro-motive force in an alternating-current circuit; -- so called because used to “boost”, or raise, the pressure in the circuit. [ Webster 1913 Suppl. ]

2. an enthusiastic supporter. [ PJC ]

3. the first stage of a multistage rocket, providing all or part of the driving force at lift-off and until the second stage ignites, whether or not the booster stage is detached after the fuel is spent. Also called booster rocket. In some cases, a booster has been used to assist the takeoff of airplanes. [ PJC ]

4. (Med.) an immunizing medication given after an initial immunizing treatment, to provide increased or extended immunity. Also called booster dose. When in the form of an injection, it may be called a booster shot. [ PJC ]

5. any auxiliary device or part of a device to provide increased power, force, or effectiveness, as with a pumping device. [ PJC ]

6. (Mil.) an explosive charge which is more powerful than a primer, used in conjunction with a primer to assure detonation of the main charge. Integrated blasting caps containing separate compartments for primer and booster are used as detonators for certain explosives. [ PJC ]