| Term | Definition |
| System - | A collection of matter or space enclosed within prescribed boundaries. |
| Boundary - | An analytical device and is either a real or imaginary surface
which separates that which is chosen to be the system from that which has
been chosen to be excluded from the system, i.e. The Surroundings.
A boundary defines the system; or the matter or space which constitutes the system; everything outside the boundary constitutes the surroundings. |
| Surroundings- | Anything outside the boundary. |
| State- | The condition of the system.
A state of a system is identified by the values of the properties of the system. |
| Property- | Any quantity whose change, as
the system passes from some initial state to some final state, is solely
dependent on the values of that quantity at the initial state and the value
of the quantity at the final state. A property of a system is an observable quantity which is definable without reference to the history of the system. Any measure may be considered a thermodynamic property of the system if and only if it is a characteristic quality of the entire system. |
| Energy- | Concept which helps us understand physical phenomenon, but difficult to define. The meaning emerges from a fundamental principle, The First Law of Thermodynamics. |
| Process- | A process performed on a system or by a system causes a change in the state of the system. Conversely, if a system changes state some process has been performed on the system or by the system. Since there has been a change of state at least some of the properties of the system must change whenever a process occurs. A process may or may not include interactions with the surroundings. |
| Path- | A path is described by the infinite number of states through which a system passes in a process. In an analysis, it is highly desirable for this path to be known. Though a path is often approximated, it is seldom if ever possible to know it. |
| Cycle- | A collection of processes for which the initial and final state of the system are identical. |
| Isolated System- | A system that does not exchange matter or energy, in any form, with its surroundings. |
|
Closed System- |
A system that exchanges only energy
with its surroundings. The mass of the system is constant. |
|
Open System- |
A system that exchanges matter as well as energy with its surroundings. |
| Phase- | A quantity of matter that is homogeneous in chemical composition and physical structure. |