在数学领域中，两个集合 A 和 B 是等势的(equinumerous)，当它们有相同的势的时候，就是说如果存在一个双射 f : A → B。这通常指示为

势的研究中经常叫做等势性(equinumerosity)。有时还使用术语 equipotent 或 equipollent。

在集合范畴中，带有函数作为态射的所有集合的范畴，在两个集合之间的同构正好是一个双射，而两个集合正好是等势的，如果它们在这个范畴中是同构的。

In mathematics, two sets A and B are equinumerous if there exists a one-to-one correspondence (a bijection) between them, i.e. if there exists a function from A to B such that for every element y of B there is exactly one element x of A with f(x) = y. Equinumerous finite sets have the same number of elements. The definition of equinumerosity using bijections can be applied to both finite and infinite sets and allows one to state whether two sets have the same size even if they are infinite. Unlike finite sets, some infinite sets are equinumerous to proper subsets of themselves.