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The Cardinality of a set or dimension of data is a measure of the number of elements or possible values a set or dimension can have.

High-cardinality refers to data values that are unique or very uncommon. Examples of high-cardinality values are identification numbers, user names, UUIDs, or process IDs.

Low-cardinality refers to data with common values or values that are shared or commonly found across dimensions. Examples include boolean values, status flags, or gender classifications.