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In thermodynamics and statistical mechanics, a state function, also known as a state variable or thermodynamic property, is a property of a system that is determined by the current state of the system, independent of the path taken to reach that state. This means that the value of a state function depends only on the present condition of the system, not on its history or the sequence of processes it has undergone.

Key characteristics of state functions include that their changes are path-independent. If a system transitions from an initial state to a final state, the change in a state function is the same regardless of the intermediate steps or processes involved. This property makes them extremely useful for analyzing thermodynamic processes, as one can focus solely on the initial and final conditions.

Examples of common state functions include temperature (T), pressure (P), volume (V), internal energy (U), enthalpy (H), entropy (S), and Gibbs free energy (G). For instance, the internal energy of a gas depends only on its temperature and pressure, not on how it reached that temperature and pressure.

In contrast to state functions are path functions, whose values depend on the process or path taken by the system. The most common examples of path functions are heat (Q) and work (W). The amount of heat transferred or work done during a process can vary depending on how the system changes from its initial to its final state. This distinction between state functions and path functions is fundamental to understanding the laws of thermodynamics.