在热力学领域中，不可逆过程(Irreversible process)是相对可逆过程而言的，指的是在时间反演变换下只能单向进行的热力学过程，这种热力学过程所具有的性质被称作不可逆性。从热力学角度而言，自然界中所有复杂的热力学过程都具有宏观上的不可逆性。宏观上不可逆性现象产生的原因在于，当一个热力学系统复杂到足够的程度，组成其系统的分子之间的相互作用使系统在不同的热力学态之间演化；而由于大量分子运动的高度随机性，分子和原子的组成结构和排列的变化方式是非常难于预测的。热力学状态的演化过程需要分子之间彼此做功，在做功的过程中也伴随有能量转换以及由分子间摩擦和碰撞引起的一定热量的流失和耗散，这些能量损失是不可复原的。

In science, a process that is not reversible is called irreversible. This concept arises most frequently in thermodynamics.

In thermodynamics, a change in the thermodynamic state of a system and all of its surroundings cannot be precisely restored to its initial state by infinitesimal changes in some property of the system without expenditure of energy. A system that undergoes an irreversible process may still be capable of returning to its initial state; however, the impossibility occurs in restoring the environment to its own initial conditions. An irreversible process increases the entropy of the universe. However, because entropy is a state function, the change in entropy of the system is the same whether the process is reversible or irreversible. The second law of thermodynamics can be used to determine whether a process is reversible or not.