TITLE: What is the definition of the $\uplus$ symbol?
QUESTION [7 upvotes]: Hi,
 I have what I hope is a very simple question related to unfamiliar notation.
 I am looking through a maths paper on a topic related to set theory which contains a symbol, 
$\uplus$, 
and I would like to know how, if at all, it differs from the typical 
$\cup$ 
symbol in terms of its meaning.
 The context leads me to believe that it does not in fact differ at all but since I don't even know the name of the symbol other than the latex id that I looked up, I can't seem to confirm that suspicion.
 Cheers
edit:  it seems that the latex renderer also does not know about this obscure symbol '\cupplus' but '\uplus' does work.
edit2:  thanks for all the replies!  however, since everyone commented rather than providing an answer it seems I cannot grant the coveted 'answer' status to anyone.  the disjoint union makes the most sense.

REPLY [7 votes]: It may also denote that the union is a multi-set (and the both operands are possibly multi-sets). Then the multiplicity of an element in the result is the sum of its multiplicities in the operands, that explains the existence of the + in the symbol.