A key Ted Nelson/Xanadu 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).
A key use-case (reminder to self, maybe this isn't key case for others): in a Wiki you want to break up content into small nuggets, but this can cause you to lose a sense of context. You can make something like a Parent page, but if it just lists those actual content pages, that can be too bare a page to communicate much - and most people are less likely than me to open a whole bunch of pages in tabs right away. So bringing that content "into" the single current window if valuable. (Contra: this can be a problem when the content nodes are long.) (see Node Web)