Definition
A removable threaded plug fitted at the lowest point of an engine's oil sump (or other fluid reservoir) that, when unscrewed, allows the contained fluid to drain out for servicing.
Plain English
A screw-in plug at the bottom of the engine that you take out to let the old oil drain.
Context Anchor
Seen in engine and servicing discussions, especially when describing how oil or other fluids are drained from low points on the airplane.
Why Pilots Care
Opening the drain plug lets a pilot or mechanic remove water and sediment that would otherwise reach the engine and cause power loss or failure.
Intuition Check
Do not think of a drain plug as a loose household-style cap. In an aircraft, it is a fitted closure that belongs in a specific low point and must be secure before flight.
Example Sentence 1
After draining the oil, the mechanic installed a new gasket on the drain plug and torqued it to specification.
Example Sentence 2
After the oil change, the mechanic tightened the drain plug on the engine sump and checked for leaks.