A substance is in the aeriform state when it does not have a defined shape (it is therefore a fluid) or a defined volume, as it tends to expand, completely filling the container that contains it. Any substance above its boiling point can, therefore, be defined as aeriform. An aeriform can also exist below the boiling point, vaporizing from its liquid.
Formally, gas is aeriform that is at a temperature higher than its critical temperature (temperature above which a substance cannot exist in a liquid state), and this means that it cannot be condensed in any way by simple compression that is, it cannot be brought to the liquid state by compressing it at a constant temperature.
When, on the other hand, they are at a temperature lower than their critical temperature, the aeriforms are said to be in a vapor state; a vapor can, therefore, become liquid when it is sufficiently compressed at a constant temperature.