noun The substance constituting Earth's atmosphere: a gaseous mixture of nitrogen, oxygen, and various trace gases.
This drill runs on compressed air.
noun The substance of the atmosphere seen as an agency of freshness.
I'm going outside to get some air.
noun One of the four elements of the ancient Greeks and Romans.
noun The surrounding ambience or mood; see also in the air.
The air was tense as the judge rose to deliver his verdict.
noun A local environment or atmosphere, in the context of its effects on behavior, health, weather, etc.
There was a tension in the air which made me suspect an approaching storm.
noun The apparently open space above the ground which this substance fills, formerly thought to be limited by the firmament but now considered to be surrounded by the near-vacuum of outer space.
The flock of birds took to the air.
noun The substance of the atmosphere on a planet other than Earth.
The air on Mars is very thin and consists mostly of carbon dioxide.