The numBer of elements in a set. If two sets have the same numBer of elements (i.e. there is a bijectionBetween 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 infiniteordinal, omega (the numBer of {natural numBer}s). (1995-03-29)