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)