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

coalesced sum
smash sum

    (Or "smash sum") In domain theory, the coalesced
   sum of domains A and B, A (+) B, contains all the
   non-bottom elements of both domains, tagged to show which
   part of the sum they come from, and a new bottom element.

    D (+) E =  bottom(D(+)E) 
   	   U  (0,d) | d in D, d /= bottom(D) 
   	   U  (1,e) | e in E, e /= bottom(E) 

   The bottoms of the constituent domains are coalesced into a
   single bottom in the sum.  This may be generalised to any
   number of domains.

   The ordering is

   	bottom(D(+)E) <= v  For all v in D(+)E

   	(i,v1) <= (j,v2)    iff i = j & v1 <= v2

   "<=" is usually written as LaTeX \sqsubseteq and "(+)" as
   LaTeX \oplus - a "+" in a circle.

   (1994-12-22)