In mathematics, two sets or classes "A" and "B" are equinumerous if there exists a one-to-one correspondence (or bijection) between them, that is, 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 sets are said to have the same cardinality (number of elements). The study of cardinality is often called equinumerosity ("equalness-of-number"). The terms equipollence ("equalness-of-strength") and equipotence ("equalness-of-power") are sometimes used instead.