The cardinality of a finite set is simply the number of members in it.
Since cardinality is such a common concept in mathematics, a variety of names are in use.
British Dictionary definitions for cardinality
cardinality
/ˌkɑːdɪˈnælɪtɪ/
noun
1.
(maths) the property of possessing a cardinal number
2.
(maths, logic) (of a class) the cardinal number associated with the given class. Two classes have the same cardinality if they can be put in one-to-one correspondence
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 infiniteordinal, omega (the number of natural numbers). (1995-03-29)