Cardinality

In set theory, the cardinality of a set $A$ is a formal definition of the size of $A$. Cardinality must be a natural number, and the cardinality of $A$ is written $|A|$.

Two sets $A$ and $B$ have the same cardinality if and only if there exists a bijective function from $A$ to $B$. Then $|A|=|B|$.