Climb into Cantor’s Attic, where you will find infinities large and small. We aim to provide a comprehensive resource of information about all notions of mathematical infinity.

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Cantor's Attic (original site)
Joel David Hamkins blog post about the Attic
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Omega, $\omega$

The smallest infinite ordinal, often denoted $\omega$ (omega), has the order type of the natural numbers. As a von Neumann ordinal, $\omega$ is in fact equal to the set of natural numbers. Since $\omega$ is infinite, it is not equinumerous with any smaller ordinal, and so it is an initial ordinal, that is, a cardinal. When considered as a cardinal, the ordinal $\omega$ is denoted $\aleph_0$. So while these two notations are intensionally different—we use the term $\omega$ when using this number as an ordinal and $\aleph_0$ when using it as a cardinal—nevertheless in the contemporary treatment of cardinals in ZFC as initial ordinals, they are extensionally the same and refer to the same object.

Countable sets

A set is countable if it can be put into bijective correspondence with a subset of $\omega$. This includes all finite sets, and a set is countably infinite if it is countable and also infinite. Some famous examples of countable sets include:

The union of countably many countable sets remains countable, although in the general case this fact requires the axiom of choice.

A set is uncountable if it is not countable.