Air or another gas; the atmosphere; aircraft.
[Greek aēr, air.]
Some examples relate to the atmosphere: aerology is its study; aerobiology is the study of airborne microorganisms. Others refer to air as a gas, as in aerate, to introduce air into something. The form sometimes refers specifically to oxygen: something aerobic involves or requires oxygen from the air; aerobics is exercise that increases consumption of oxygen. It can refer more broadly to any gas: an aerosol is a fine mist dispersed in a gas (it can also be the container that dispenses it).
Aerodynamics is the study of objects moving through the air, such as aerofoils, surfaces such as wings shaped to produce lift; aeronautics is the science of aerial travel; aerospace relates to vehicles that travel both through air and outer space; an aeroshell protects a spacecraft during re-entry into the atmosphere.