A **countable set** is a set that has the same size (cardinality) as some subset of the set of natural numbers. In more formal terms, a set \( S \) is countable if there exists a bijection (a one-to-one and onto function) between \( S \) and the set of natural numbers \( \mathbb{N} \) or a finite subset of \( \mathbb{N} \).
