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)