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 foregn key if the primary key is in a DIFferent column in the _same_ table?] (2005-01-14)