Source : Free On-Line Dictionary of Computing

cardinality
     
         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
        number}s).
     
        (1995-03-29)