A charcoal-filled container installed inside the engine compartment of a gasoline-powered vehicle. When the vehicle is refueled, gasoline vapors displaced from the vehicle's tank are directed through the canister. The charcoal removes most of the hydrocarbon contaminants from the vapors before the vapors are released to the atmosphere.
Because these charcoal-filled canisters are designed for installation on the vehicle itself-onboard, so to speak, they have come to be referred to as onboard canisters.
Federal regulations require that automobiles and light-duty trucks be equipped with onboard canisters.