Definition
The second of the four strokes in a reciprocating engine's operating cycle, during which the piston moves upward in the cylinder with both intake and exhaust valves closed, compressing the fuel/air mixture into a smaller volume in preparation for ignition.
Plain English
The part of the engine cycle where the piston squeezes the fuel and air mixture tightly into a small space so it will burn powerfully when the spark plug fires.
Context Anchor
Seen in explanations of the four-stroke operating cycle of aircraft reciprocating engines.
Derivation
From Latin 'comprimere' meaning 'to press together.' The piston literally presses the mixture together into a smaller space, raising its pressure and temperature so combustion is fast and forceful.
Why Pilots Care
Adequate compression is required for efficient combustion and full engine power; weak compression causes power loss and rough running.
Analogy
It is like pushing down on a bicycle pump with the outlet blocked: the air is forced into less space and pressure builds.
Intuition Check
Do not read stroke here as a hit or medical condition. In an engine, a stroke is one complete piston movement in one direction.
Example Sentence 1
On the compression stroke, both valves are closed and the piston pushes upward, raising the pressure of the fuel/air mixture.
Example Sentence 2
A pilot checks compression by noting how firmly the engine resists turning over on the starter, which indicates the health of the compression stroke.