The Collaborative International Dictionary of English v.0.48:

Booster \Boost"er\ (b[=oo]st"[~e]r), 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]

WordNet (r) 3.0 (2006):

booster rocket
    n 1: the first stage of a multistage rocket [syn: booster,
         booster rocket, booster unit, takeoff booster,
         takeoff rocket]