According to Chap. 5 a quantity, which may meaningfully be termed “pressure,” is required to exist, at least in equilibrium. This variable relies on a process, in which some extensive quantity changes while entropy remains invariant (adiabatic processes). Here a definition of pressure based on the quantum microstate is given. On the basis of this definition the above-mentioned behavior is demonstrated for various standard processes. It is shown that the adiabatic process may even be stabilized by the influence of the environment.