Natural Key
An Attribute that occurs in the real world and has meaning that can uniquely identify a Tuple in a Relation. For instance Email Address in a user table could be used as a Natural Primary key.
Example:
| UserName | Password | |
|---|---|---|
| AJones | alex.jones@abc.com | ######## |
| BSmith | bob.smith@def.com | ######## |
| CBloggs | chris.bloggs@xyz.com | ######## |
In the table above UserName could be used as a unique identifier as all entries are unique. It is a Natural Key because User Name has real world meaning.