When a system is at equilibrium under a given set of conditions, it is said to be in a definite thermodynamic state. The state of the system can be described by a number of state quantities that do not depend on the process by which the system arrived at its state. They are called intensive variables or extensive variables according to how they change when the size of the system changes. The properties of the system can be described by an equation of state which specifies the relationship between these variables. State may be thought of as the instantaneous quantitative description of a system with a set number of variables held constant.
A thermodynamic process may be defined as the energetic evolution of a thermodynamic system proceeding from an initial state to a final state. It can be described by process quantities. Typically, each thermodynamic process is distinguished from other processes in energetic character according to what parameters, such as temperature, pressure, or volume, etc., are held fixed; Furthermore, it is useful to group these processes into pairs, in which each variable held constant is one member of a conjugate pair.
Several commonly studied thermodynamic processes are:
* Adiabatic process: occurs without loss or gain of energy by heat
* Isenthalpic process: occurs at a constant enthalpy
* Isentropic process: a reversible adiabatic process, occurs at a constant entropy
* Isobaric process: occurs at constant pressure
* Isochoric process: occurs at constant volume (also called isometric/isovolumetric)
* Isothermal process: occurs at a constant temperature
* Steady state process: occurs without a change in the internal energy