Plain English Meaning
A small unit on the vacuum pump that pulls the oil out of the air the pump pushes out, sends the oil back to the engine, and lets the cleaned air exit the aircraft.
Full Definition
A device fitted to a wet-type vacuum pump that separates lubricating oil from the air discharged by the pump, returning the oil to the engine sump and venting clean air overboard.
Context Anchor
Seen in wet-type vacuum pump diagrams, usually on the outlet side of the pump before the air is vented overboard.
Why Pilots Care
Keeps oil from contaminating the cockpit and ensures the pump's lubricating oil is recycled to the engine.
Intuition Check
An oil separator is not an oil filter for cleaning all engine oil. Here, it separates oil from the air leaving the wet-type vacuum pump.