余熵是处于非平衡态的物质与其处于完整晶体状态的物质在温度趋近于绝对零度时熵的差值。这一概念在凝聚体物理学领域主要用来描述玻璃态非晶体及塑性晶体与完整晶体相比在绝对零度时的熵，而完整晶体的熵根据热力学第三定律在绝对零度时为零。当物质冷却时有多种状态时，这一概念会被常常用到。最为常见的非平衡状态为玻璃态。

一个较为常见的例子是一氧化碳，其分子有非常小的电偶极矩。当一氧化碳晶体被冷却至接近绝对零度时，极少数的一氧化碳分子会有足够时间按照完整晶体形态排列（所有分子的排列方向一致）。这样整个晶体内部就有种不同的微观状态，可以得出此时的熵。

Residual entropy is the difference in entropy between a non-equilibrium state and crystal state of a substance close to absolute zero. This term is used in condensed matter physics to describe the entropy at zero kelvin of a glass or plastic crystal referred to the crystal state, whose entropy is zero according to the third law of thermodynamics. It occurs if a material can exist in many different states when cooled. The most common non-equilibrium state is vitreous state, glass.