Countable set

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If you can count the things in a set, it is called a countable set. A set with one thing in it is countable, and so is a set with one hundred things in it. A set with all the natural numbers (counting numbers) in it is countable too. It's infinite but if someone counted forever they wouldn't miss any of the numbers out. Sometimes when people say 'countable set' they mean countable and infinite. Georg Cantor coined the term.