A key Ted Nelson/Xana Du reference. I'm not clear on the precise definition. I think of it as "inclusion via reference" (e.g. from the viewer's perspective a remote document is "part of" the current document, but it's really being pulled/displayed from that remote original store).
aka Trans Clusion - see ChrisDent:TransClusion, EugeneEricKim:TransClusions, TransClusion
any relation to XInclude/Xml Inclusions? http://www.w3.org/TR/xinclude/