Definition:Countable Set

Definition
A set $X$ is said to be countable if there exists an injection $f: X \to \N$.

That is, if it can be shown that it is possible to exhaustively count its elements.

An infinite set is described as countably infinite if it is countable, and is described as uncountable otherwise.

The cardinality of a countably infinite set is denoted by the symbol $\aleph_0$ (aleph null).

From Infinite Set has Countable Subset it is seen that $\aleph_0$ is the smallest possible cardinality of an infinite set.

Also known as
The words denumerable and enumerable are sometimes encountered. They mean the same thing as countable but usually imply that the set is infinite. However, these terms are also going out of fashion.

Some sources use countable to describe a set which has exactly the same cardinality as $\N$.

That is, $X$ is said under this criterion to be countable iff there exists a bijection from $X$ to $\N$, i.e. equivalent to $\N$.

However, this definition seems to be going out of fashion, as the very concept of the term countable implies that a set can be counted, which, plainly, a finite set can be.

Also see

 * Uncountable
 * Sufficient Conditions for Uncountability