Definition
One of the three states of matter (solid, liquid, gas), characterized by molecules that move freely and independently, with no fixed shape or volume. A gas expands to fill the entire container that holds it and can be compressed or expanded by changes in pressure and temperature.
Plain English
A substance, like air, that has no shape of its own and spreads out to fill whatever space it is in. You can squeeze it into a smaller space or let it expand into a larger one.
Context Anchor
Seen in discussions of engines, weather, breathing oxygen, fuel vapor, combustion, and the behavior of air around an aircraft.
Derivation
From the Dutch word 'gas,' coined in the 1600s by chemist J.B. van Helmont, based on the Greek 'chaos' meaning 'empty space.' That origin captures the idea well: a gas behaves like loose, free-moving matter spread through space rather than holding a set form.
Why Pilots Care
Almost everything a pilot deals with -- lift, drag, engine performance, altimetry, weather -- depends on the behavior of gases. Understanding that gases compress, expand, and change pressure with temperature is the foundation for understanding how aircraft and the atmosphere work.
Grounding Statement
If air leaks from a tire, it does not pour out like water; it spreads into the surrounding space because it is a gas.
Intuition Check
Do not assume “gas” always means gasoline. In technical aviation use, “gas” often means an airlike form of matter, such as air, vapor, exhaust, or oxygen.
Example Sentence 1
As the aircraft climbs, the air -- a mixture of gases -- becomes less dense, which reduces engine power and lift.
Example Sentence 2
This Cessna requires 100LL gas only.