**Definition :** The size of a set.

The cardinality of a set is the number of unique elements in the set. For example the cardinality of {1,3,4,5} is 4 while the cardinality of {1,3,4,8,9} is 5. Two sets have the same cardinality if there exists a bijective function between them.

**See Also :** Bijection (Function)