Half-life (t½) is the time required for the concentration of a reactant to fall to half its initial value. How the half-life depends on concentration distinguishes the reaction orders: it is constant for a first-order reaction, but depends on the initial concentration for zero- and second-order reactions. It is widely used, for example, to describe radioactive decay and drug elimination.

半衰期(t½)是反应物浓度降至其初始值一半所需的时间。半衰期对浓度的依赖关系可区分反应级数:对一级反应它恒定,而对零级和二级反应则取决于初始浓度。它被广泛使用,例如用于描述放射性衰变和药物的消除。