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 USA 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 a 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 "one version of the truth" can be verified, because the natural key is based on a real-world observation.
The main disadvantage of choosing a natural key is that its value may change and the relational database engine may not be able to propagate that change across the related foreign keys. For example, if person_name is used as the primary key for the person table, and a person gets married and changes name, then all of the one-to-many related tables need to be updated also. The secondary disadvantage of choosing a natural key is identifying uniqueness. The primary key must consist of the attributes that uniquely identify a row. However, it may be difficult (or it may add constraints) to create a natural key on a table. For example, if person_name is used as a primary key for the person table, many persons may share the same name and all but the first entry will be rejected as a duplication. The uniqueness constraint may be overcome by adding an additional column to the primary key, like street_address, to increase the likelihood of uniqueness.