Borel hierarchy

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What Wikipedia says

In mathematical logic, the Borel hierarchy is a stratification of the Borel algebra generated by the open subsets of a Polish space; elements of this algebra are called Borel sets. Each Borel set is assigned a unique countable ordinal number called the rank of the Borel set.

Excerpt from the Wikipedia article “Borel hierarchy”, under the CC BY-SA 4.0 license. Read the article on Wikipedia