Webster's English Dictionary

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accelerator

ac.cel.er.a.tor \ik-'sel-*-.ra-t-*r, ak-\ n : one that accelerates : as  : 
   a muscle or nerve that speeds the performance of an action  : any of 
   several devices for increasing the speed of a motor vehicle engi ne; esp : 
   a foot-operated throttle that varies the supply of fuel-air mixture to the 
   combustion chamber : a substance that speeds a chemical reaction  : an 
   apparatus for imparting high velocities to charged particles