A candidate key made up of multiple fields
In Relational Databases, a Composite Key (sometimes called “Compound Key”) is a Candidate Key that is comprised of more than 1 attribute.
Example
Given a table with fields:
student_idfirst_namelast_nameis_enrolled
There are two sets of Candidate Keys.
student_idfirst_name,last_name
Since the Candidate Key student_id is comprised of only one attribute, it is NOT a composite key. Also, because student_id is
Since the Candidate Key first_name, last_name is comprised of more than one attribute, it is a Composite Key.
Source
Database normalization - Wikipedia