In relational model database design, a natural key is a key that is formed of attributes that already exist in the real world. For example, a US citizen's social security number could be used as a natural key. In other words, a natural key is a candidate key that has a logical relationship to the attributes within that row. A natural key is sometimes called domain key.
The main advantage of a natural key over a surrogate key, which has no meaning outside the database environment, is that it already exists; there is no need to add a new, artificial column to the schema. Using a natural key (when one can be identified) also simplifies data quality: It ensures that there can only be one row for a key; this single version of the truth can be verified because the natural key is based on a real-world observation.