## 2.1. Countable Infinity

### Definition 2.1.3: Countable and Uncountable

If a set

**A**has the same cardinality as**N**(the natural numbers), then we say that**A**is**countable**. In other words, a set is countable if there is a bijection from that set to**N**.
An alternate way to define countable is: if there is a way to **enumerate**
the elements of a set, then the set has the same cardinality as **N** and
is called countable.

A set that is not countable (and not finite) is called **uncountable**.