Definitions of thermodynamic terms

Definitions for thermodynamics ^ref.

Universe: Everything of importance which is outside the system in question. This is sometimes thought of as an isolated system which includes another system which is being addressed. Discussion

Surroundings: That portion of the universe which does not include the system in question. Thus, a universe consists of a system and its surroundings.

Spontaneous process: A process which requires no influence from outside the system to proceed. A process which can proceed in an isolated system.

Nonspontaneous process: A process which does require influence from outside the system to proceed. This process is impossible in an isolated system.

Equilibrium: A condition in which there is no net change in any of the state functions of a system which has been allowed to arrive at rest with its surroundings.