foreign key definition database
in a database table
containing values that are also found in some primary key
column (of a different table). By extension, any reference to entities of a different type.
Some RDBMSs allow a column to be explicitly labelled as a foreign key and only allow values to be inserted if they already exist in the relevant primary key column.
[Is it still a foreign key if the primary key is in a different column in the __same__ table?]