A column 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?]