The cardinal of a set is the number of elements in a set.
Expression that is sometimes used to refer to the class of sets equipotent to a given set.
A cardinal number is a number that characterizes the quantity of elements in a set, as opposed to the ordinal number that characterizes a rank in a list.
Example
Consider this set: E = {1, 2, 3, 4, 5, 6}. The cardinal of the set is 6.