耗散是出现在非匀相热力学系统中不可逆过程的结果。耗散过程是指能量（内能、动能或势能）由一种形式转换到另一种形式，而且后者可以作的功少于前者。例如将能量转换为热是一种耗散过程，因为热会由较热的物体转移到较冷的物体，二者的温度差会减少，根据热力学第二定律，这様会使二者物体所组成系统可作的机械功减少。

热力学的耗散过程在本质上就是不可逆的，此过程以固定的速率产生熵。若一个过程的温度都有明确的定义，单位体积内温度乘以熵的变化率即为单位体积耗散的能量。

不可逆过程包括：

一些特定情形下的耗散过程无法用单一的哈密顿力学方程来描述。耗散过程需要将允许的哈密顿描述形成一个集合，每一个描述一个特定未知的相关物理量，包括摩擦力其他类似使能量减少的力，也就是将相干性的能量转换到各向同性较高的能量分布的力。

Dissipation is the result of an irreversible process that takes place in inhomogeneous thermodynamic systems. A dissipative process is a process in which energy (internal, bulk flow kinetic, or system potential) is transformed from some initial form to some final form; the capacity of the final form to do mechanical work is less than that of the initial form. For example, heat transfer is dissipative because it is a transfer of internal energy from a hotter body to a colder one. Following the second law of thermodynamics, the entropy varies with temperature (reduces the capacity of the combination of the two bodies to do mechanical work), but never decreases in an isolated system.