Definition
The condition of a substance, usually a gas, that has been forced into a smaller volume than it would naturally occupy at ambient pressure, resulting in higher pressure and density within that volume.
Plain English
Squeezed into a smaller space, which raises its pressure.
Context Anchor
Seen in engine, air system, oxygen system, and weather discussions where air or gas is squeezed into a smaller space.
Derivation
From the Latin 'comprimere', meaning 'to press together' (com- 'together' + premere 'to press'). The aviation meaning is the literal one: the substance has been pressed together into a smaller volume.
Why Pilots Care
Adequate compression produces the high pressure needed for reliable ignition and power; loss of compression reduces performance and signals engine wear.
Analogy
A bicycle pump compresses air when you push the handle down. The air is squeezed into less space, so it comes out with more force.
Grounding Statement
If air is squeezed into a smaller space, it is compressed and its pressure goes up.
Intuition Check
Do not think of compressed as just “shortened” or “condensed wording.” In aviation, compressed usually means physically squeezed into a smaller space, especially air or gas.
Example Sentence 1
The aircraft's emergency gear extension system uses compressed nitrogen to push the landing gear down if the hydraulic system fails.
Example Sentence 2
The ground crew used compressed air to spin the starter on the turbine engine.