Plain English Meaning
A setting that lets one engine drink fuel from the tank on the other side of the airplane, instead of from its own side.
Full Definition
A fuel system configuration in a multi-engine airplane in which fuel from a tank on one side of the airplane is routed across to feed an engine on the opposite side. It is selected using the crossfeed valve and is used to manage fuel imbalance between left and right tanks, or to keep an engine running after a fuel system problem on its own side.
Context Anchor
Seen in multiengine airplane fuel-system procedures, especially when managing fuel balance or handling an engine or fuel-supply problem.
Derivation
From 'cross' (across) and 'feed' (to supply with fuel). The name describes exactly what it does: feeding an engine from across the airplane.