<mathematics> The number of elements in a set; intuitively, the set's "size" or "magnitude". Notation: a double bar over the symbol denoting the set; also "|S|" and "card S" when S is the symbol denoting the set. If two sets have the same number of elements (i.e. there is a bijection between them) then they have the same cardinality. A cardinality is thus an isomorphism class in the category of sets.
aleph 0 is defined as the cardinality of the first infinite ordinal, omega (the number of natural numbers).
[FOLDOC] and [Glossary of First-Order Logic]
Try this search on OneLook / Google