a pedal that controls the throttle valve
"he stepped on the gas"
a valve that regulates the supply of fuel to the engine
"The driver adjusted the accelerator valve by hand to manually regulate the fuel supply and keep the old engine running smoothly without an electric pedal."
(chemistry) a substance that initiates or accelerates a chemical reaction without itself being affected
"The catalyst acts as an accelerator, speeding up the decomposition of hydrogen peroxide while remaining chemically unchanged."
a scientific instrument that increases the kinetic energy of charged particles
"The massive accelerator at CERN speeds up protons to nearly the speed of light before colliding them to study subatomic structures."
One who, or that which, accelerates.
"The car's accelerator pedal is designed to increase speed when pressed down."
In plain English: An accelerator is something that makes things happen faster, like pressing a pedal to make a car go quicker.
"The driver pressed down on the accelerator to help the car reach highway speed quickly."
The word accelerator comes from the Latin verb accelerare, meaning "to hasten." It originally referred to the act of speeding up before being applied to motor vehicles in the early 20th century.