Plain English Meaning

A lubrication setup where the engine stores its own oil inside the bottom of the engine, rather than in a separate tank. The oil keeps cycling through the engine and falling back into the same built-in pool.

Full Definition

An engine lubrication system in which the oil supply is carried in a reservoir that is part of the engine itself, typically the lower portion (sump) of the crankcase. A pump draws oil from this internal reservoir, circulates it through the engine, and the oil drains back into the sump by gravity to be used again.

Context Anchor

Seen in engine-system discussions, preflight knowledge, and oil servicing for many light training aircraft.

Derivation

‘Sump’ comes from a Middle English word meaning a pit or pool where liquid collects. ‘Wet’ here simply means the sump holds oil while the engine runs — the storage is wet with the working oil supply.