cardinality

In mathematics, the cardinality of a set is a measure of the "number of elements of the set". There are two approaches to cardinality – one which compares sets directly using  bijections and injections, and another which uses cardinal numbers.
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<mathematics> The number of elements in a 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).
(1995-03-29)