Natural Key

A natural key is a column that already exists in the real-world data and uniquely identifies a row — like a national ID number, ISBN, or email address — as opposed to an artificially generated surrogate key.

Natural keys carry real meaning, which can be convenient for lookups, but they come with risk: if the business ever needs to correct or reissue that value, every foreign key referencing it must be updated too. That's the main reason most schemas use a surrogate key for the primary key and keep the natural key as a separately unique-constrained column.