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

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)