Plain English Meaning
Gas squeezed into a bottle at high pressure. When you open a valve, the gas rushes out and that force can push something — like pushing the landing gear down.
Full Definition
A gas held under pressure inside a sealed container, storing energy that can be released to perform mechanical work such as extending landing gear when normal hydraulic systems fail.
Context Anchor
Seen in emergency landing gear extension systems, especially where a backup gas bottle is used to help extend the gear.
Derivation
From Latin comprimere, 'to press together.' The gas has been pressed into a smaller space than it would naturally occupy, which stores energy. When released, it expands out and that is what does the work.