Definition
An engine lubrication system in which the oil supply is held in a separate external tank rather than in a reservoir built into the bottom of the engine. A pump draws oil from the tank, circulates it through the engine, and a second pump (the scavenge pump) returns the used oil from the engine back to the tank.
Plain English
The engine's oil is stored in its own tank outside the engine, not in a pan underneath it. Oil is pumped from the tank into the engine, then pumped back out to the tank again.
Context Anchor
Seen in aircraft engine oil system descriptions and during preflight oil quantity checks on engines that use a separate oil tank.
Derivation
"Sump" comes from the Middle English word for a pit or pool where liquid collects. A "wet sump" keeps oil pooled inside the engine itself; a "dry sump" keeps the engine's oil pan dry by sending the oil out to a separate tank.
Why Pilots Care
Maintains reliable oil supply and pressure during inverted or aggressive maneuvers that would starve a wet-sump system.
Analogy
Think of it like a small machine fed by a separate oil container. The oil goes into the machine to do its job, then is collected and sent back to the container instead of being stored inside the machine.
Intuition Check
Do not read “dry” as “no oil.” In this term, “dry” means the bottom of the engine is not the main oil reservoir.
Example Sentence 1
Most radial engines use a dry-sump oil system because the lower cylinders would otherwise sit submerged in oil.
Example Sentence 2
Before flight the pilot checks the oil quantity in the external tank of the dry-sump oil system.