Plain English Meaning
A heat-resistant wall built between the engine and the rest of the aircraft. Its job is to keep an engine fire from spreading into where the people sit.
Full Definition
A fire-resistant bulkhead in an aircraft that separates the engine compartment from the rest of the airframe, designed to prevent flames, heat, and combustion gases from passing into the cockpit or cabin in the event of an engine fire.
Context Anchor
Seen during preflight inspection, maintenance, aircraft construction, and discussions of engine compartment fire protection.
Derivation
From the literal idea of a 'wall against fire.' The term predates aviation and was originally used in buildings and early automobiles to describe a barrier protecting occupants from engine or furnace fires. Aviation adopted the same word for the same purpose.