Plain English Meaning
A handheld bottle in the cockpit that sprays a fire-killing chemical when you squeeze the trigger. It's there to put out small fires inside the airplane.
Full Definition
A portable device carried in the aircraft that discharges a chemical agent (commonly Halon or a Halon replacement) to suppress a fire by interrupting combustion, smothering the flames, or cooling the fuel source. Aircraft fire extinguishers are sized for cabin and cockpit use and are intended for small, contained fires such as electrical, upholstery, or minor cabin fires.
Context Anchor
You encounter this term in aircraft equipment checks and in emergency procedures for smoke or fire in the cabin.
Derivation
From Latin extinguere, 'to put out' or 'quench.' The word literally describes a tool that puts out fire. Knowing the reinforces that its only job is to stop a fire that has already started -- it is not a preventive device.