The Free On-line Dictionary of Computing (30 December 2018):

relational language

    Any kind of programming language that specifies
   output in terms of some property and some arguments.  For
   example, if Tom has two brothers, Dick and Harry, a relational
   language will respond to the query "Who is Tom's brother?"
   with either Dick or Harry.  Notice that unlike functional
   languages, relational languages do not require a unique
   output for each predicate/argument pair.  Prolog is the
   best known relational language.

   (2004-05-17)