Plain English Meaning
A switch or valve that lets one engine draw fuel from the tank on the other side of the airplane.
Full Definition
A fuel system arrangement, controlled by a crossfeed valve, that allows fuel from one tank or side of the airplane to be fed to an engine on the opposite side. It is used in multi-engine airplanes to balance fuel load between tanks or to supply a working engine from a tank that would normally feed the other engine.
Context Anchor
Seen in fuel system procedures, especially in multiengine airplanes, when managing which tank is supplying which engine.
Derivation
Cross (from one side to the other) plus feed (to supply). The name describes exactly what the system does: it feeds fuel across the airplane from one side to the other.
Why Pilots Care
Enables continued engine operation if a tank is or empty and helps lateral fuel balance to avoid handling problems.