Source : Free On-Line Dictionary of Computing

relational language
     
         Relational languages specify 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 language}s, relational
        languages do not require a unique output for each
        {predicate}/argument pair.  {Prolog} is the best known
        relational language.
     
        (1995-03-31)