Definition
A combined component on a piston aircraft engine that serves two functions: the cap seals the opening through which engine oil is added, and the dipstick attached to it is a marked metal rod used to measure the quantity of oil in the engine sump.
Plain English
It is the lid you unscrew to pour oil into the engine, with a measuring stick attached so you can check how much oil is in there.
Context Anchor
Seen during preflight inspection when checking the engine oil level before starting the airplane.
Derivation
Dipstick comes from the simple action of dipping a stick into a liquid to measure its level. The name describes exactly what the tool does.
Why Pilots Care
Maintaining the correct oil level prevents engine damage from insufficient lubrication and cooling while confirming the system is sealed against leaks.
Intuition Check
Do not think of the cap as just a cover. On many airplanes, the oil filler cap and dipstick are also the pilot’s normal way to confirm the engine has enough oil before flight.
Example Sentence 1
During preflight, the pilot opened the cowling access door, removed the oil filler cap and dipstick, wiped it clean, reinserted it, and checked that the oil level was between the minimum and maximum marks.
Example Sentence 2
After topping off the oil the pilot wiped and reinstalled the oil filler cap and dipstick to ensure a proper seal.