Plain English Meaning

The engine fires in the wrong place — back through the intake instead of inside the cylinder — making a loud pop. It usually means the fuel mixture is off or the timing is wrong.

Full Definition

An abnormal combustion event in which the fuel-air mixture ignites in the engine's intake system rather than inside the cylinder, producing a sharp popping or banging sound out the carburetor or induction side. It typically occurs when the mixture is too lean or when ignition timing allows a flame to travel back through an open intake valve.

Context Anchor

Seen in hand propping because a backfire can happen while someone is pulling the through to the engine by hand.