数学上，恒等式是指等式中无论其变量如何取值，等号两边永远相等的数学式。恒等式中的等号可以用恒等号（≡）表示。

In mathematics an identity is an equality relation A = B, such that A and B contain some variables and A and B produce the same value as each other regardless of what values (usually numbers) are substituted for the variables. In other words, A = B is an identity if A and B define the same functions. This means that an identity is an equality between functions that are differently defined. For example (a + b)  =  a + 2ab + b and cos(x) + sin(x) = 1 are identities. Identities are sometimes indicated by the triple bar symbol ≡ instead of =, the equals sign.