生物半衰期是一个物质（如代谢物、药、消息分子、放射性核种）失去一半的药理、生理、或放射性效应所需的时间。通常这个词用来描述肝、肾或排泄过程将物质自身体中清除的效率。在临床上，半衰期可以用来描述血浆中某个物质的浓度减半的时间（血浆半衰期）。取决于那是什么物质，生物半衰期和血浆半衰期的关系可能非常复杂，有的物质可能在组织中累积、和血浆蛋白结合、和代谢物或受器交互。

生物半衰期是药代动力学的重要参数，常用的代表符号是。

有别物放射性同位素衰变的半衰期，生物半衰期的反应速率并不是固定的常数，而是依据更复杂的化学动力学，可以用速率方程描述。

The biological half-life or terminal half-life of a substance is the time it takes for a substance (for example a metabolite, drug, signalling molecule, radioactive nuclide, or other substance) to lose half of its pharmacologic, physiologic, or radiologic activity, as per the MeSH definition. Typically, this refers to the body's cleansing through the function of kidneys and liver in addition to excretion functions to eliminate a substance from the body. In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve (plasma half-life) its steady-state. The relationship between the biological and plasma half-lives of a substance can be complex depending on the substance in question, due to factors including accumulation in tissues (protein binding), active metabolites, and receptor interactions.