在数学里，一个具有加法运算的集合中的加法单比特，是指不论它加上任何一个在此集合内的元素x都会等于x的元素。

• 初等数学中所熟悉的加法单比特为0。

如：

5 + 0 = 5 = 0 + 5.

• 在自然数N和其所有的父集(整数Z、有理数Q、实数R、复数C)内，其加法单比特皆为0。所以对于任何一个数n，

n + 0 = n = 0 + n.

In mathematics the additive identity of a set which is equipped with the operation of addition is an element which, when added to any element x in the set, yields x. One of the most familiar additive identities is the number 0 from elementary mathematics, but additive identities occur in other mathematical structures where addition is defined, such as in groups and rings.