Definition:Countable Set

A set $$X$$ is said to be countable if there exists an injective function $$f$$ from $$X$$ to $$\N$$; that is, if it can be shown that it is possible to exhaustively number 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.

Alternative terms
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.