The state of a system defines macroscopic properties like composition or structure, energy, temperature, pressure, and volume. Changes in the state of a system can be observed or measured. For example, thermodynamics is the study of the flow and interconversion of energy, including heat.

State functions are determined by the initial and final state of the system regardless of how those states are achieved. This is convenient because it only requires two measurements or observations. Properties that are not state functions require detailed knowledge of the property change.