Gas:A compressible fluid that fills any container in which it is confined.
Technically, a gas will not condense when it is compressed and cooled, because a gas can exist only above the critical temperature for its particular composition.
Below the critical temperature, this form of matter is known as a vapor, because liquid can exist and condensation can occur.
Sometimes the terms “gas” and”vapor” are used interchangeably.
The latter, however, should be used for those streams in which condensation can occur and that originate form, or are in equilibrium with, a liquid phase.