Plain English Meaning

The engine stores its own oil inside itself, in a built-in pan at the bottom. Oil is pumped up through the engine, does its job, then drains back down into that same pan to be used again.

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 of the crankcase (the sump). A pump draws oil from this internal reservoir, circulates it through the engine to lubricate and cool moving parts, and returns it directly to the same sump by gravity.

Context Anchor

Seen in aircraft engine oil system discussions, preflight oil checks, and Pilot’s Operating Handbook information for many small training airplanes.

Derivation

‘Sump’ comes from a Middle English/Low German word meaning a pit or pool where liquid collects. ‘Wet’ here means the sump actually holds the oil supply — as to a ‘dry-sump’ system where the sump is kept empty and oil is stored in a separate external tank.