多方过程是热力学过程的一种，服从以下关系式：

其中P是压强，V是体积，n是任意一个实数（多方指数），C是一个常数。这个方程可以用来准确地描述一定的热力学系统的特征，主要是气体的膨胀或压缩。

A polytropic process is a thermodynamic process that obeys the relation:

where p is the pressure, v is specific volume, n is the polytropic index (any real number), and C is a constant. All processes that can be expressed as a pressure and volume product are polytropic processes. Some of those processes (n=0,1,), are unique. This equation can accurately characterize a very wide range of thermodynamic processes, that range from n=0 to n= which covers, n=0 (isobaric), n=1 (isothermal), n=γ (isentropic), n= (isochoric) processes and all values of n in between. Hence the equation is polytropic in the sense that it describes many lines or many processes. In addition to the behavior of gases, it can in some cases represent some liquids and solids. The polytropic process equation is particularly useful for characterizing expansion and compression processes which include heat transfer. The one restriction is that the process should display a constant energy transfer ratio K during that process: