A process is said to be adiabatic if heat neither enters nor leaves during the process. This is an important category of process.

Many real processes, especially rapid ones, may be approximated by the corresponding adiabatic process. For example the "correct" formula for the speed of sound in air was only devised when the assumption was made that the compressions and rarefactions of the sound waves were taking place adiabatically.

An "adiabat" is a line showing, for example, the relation between the pressure and the volume of a gas undergoing a reversible, adiabatic change. It contrasts with an "isothermal"