(1) A field or set of fields in a dependent file of a constraint relationship. Each foreign key value must either match a parent key value in the related parent file or be null.
(2) A column or set of columns that refers to a parent key.
(3) In a relational database, a key in one table that references the primary key in another table. See also constraint, primary key, unique key.
(4) In a federated system, a key in one nickname that references the primary key in another nickname and that the optimizer uses to improve query performance. This key is not validated when operations such as insert and update are performed.