函数的次可加性(subadditivity)是函数的一个性质，它粗略的声称计算函数对定义域中两个元素的和总是返回小于等于这个函数对每个元素的值的和的某个值。在数学的各个领域中有很多次可加函数的例子，特别是范数和平方根。加性函数是次可加函数的特殊情况。

一个函数f:A→B，其定义域A和陪域B上分别定义了某种加法和，且陪域B上定义了偏序关系“”。若该函数满足：∀x,y∈A，有。则称f对于和满足次可加性。在上下文对于和都很明确的情况下，通常简称为 f 满足次可加性，亦称f为次可加函数。

若上述函数f满足：∀有限集，有，则称f满足有限次可加性。

In mathematics, subadditivity is a property of a function that states, roughly, that evaluating the function for the sum of two elements of the domain always returns something less than or equal to the sum of the function's values at each element. There are numerous examples of subadditive functions in various areas of mathematics, particularly norms and square roots. Additive maps are special cases of subadditive functions.