Two sets are equinumerous when a bijection exists between them. This is equality of their cardinalities.
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Equinumerosity is a concept in mathematics, particularly in set theory, that refers to the property of two sets having the same cardinality, or the same "number of elements." Two sets \( A \) and \( B \) are said to be equinumerous if there exists a one-to-one correspondence (or bijection) between the elements of the sets.