Source : Free On-Line Dictionary of Computing

discriminated union
     
         The discriminated union of two sets A and B is
     
        	A + B = {(inA, a) | a in A} U {(inB, b)| b in B}
     
        where inA and inB are arbitrary tags which specify which
        summand an element originates from.
     
        A {type} (especially an {algebraic data type}) might be
        described as a discriminated union if it is a {sum type} whose
        objects consist of a tag to say which part of the union they
        belong to and a value of the corresponding type.
     
        (1995-04-25)