A thermodynamic system is the matter or region chosen for analysis. Everything outside it is the surroundings, and the surface separating them is the boundary.

A closed system contains a fixed amount of matter: energy may cross its boundary, but mass does not. An open system, often analyzed as a control volume, allows mass as well as energy to cross the boundary.

A system is described macroscopically by properties such as pressure, temperature and volume. A collection of property values that completely describes the system at an instant is its state.

A property depends only on the state, not on the path used to reach it. For example, if a gas begins and ends with the same equilibrium pressure, temperature and volume, those final property values do not record which sequence of intermediate states was followed.

A system is in thermodynamic equilibrium when there are no unbalanced macroscopic tendencies to change: temperature and pressure are uniform where equilibrium requires them, and no spontaneous macroscopic phase redistribution is occurring.

A process is a change from one state to another. A cycle is a sequence of processes that returns the system to its initial state, so every state property has zero net change over the cycle.