Definition:Cardinality of Finite Class/Definition 1

Definition
Let $A$ be a class. Let $A$ be such that:
 * there exists a bijection $\phi$ from $A$ to $n$

where $n$ is a natural number as defined by the von Neumann construction.

Then $A$ has cardinality $n$.

Also see

 * Equivalence of Definitions of Cardinality of Finite Class