等温过程是热力学过程的一种，其中系统的温度不变：ΔT = 0。一个系统与外界的热源（热浴）接触，而过程进行得足够缓慢，使得系统不断通过热交换把温度调整为与热源的温度相同，则这个过程就是等温过程。还有另一种情况，如果系统与外界没有热交换（Q = 0），则称为绝热过程。

考虑一个理想气体，温度只与内能有关，是分子的平均动能的函数。如果内能不变，温度也不变。设物质的量（n）为常数。

根据理想气体状态方程，这意味着：

所以

其中和是初状态的压强和体积，和是末状态的压强和体积，变量P和V分别表示等温过程中任何状态的压强和体积。

An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange. In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0). In other words, in an isothermal process, the value ΔT = 0 and therefore ΔU = 0 (only for an ideal gas) but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.