Boundary dissolution, for me, means things that I once conceived of as separate entities (such as "self" and "other") have been shown to correspond, be connected to each other, or comprise the same substance in some previously unforeseeable way.
I think what people call "ego death" falls into the broader category of boundary dissolution: it has everything to do with dissolving some given boundaries (in this case, those that are imposed by the normal functioning of the ego), but these are by no means the only boundaries that can be dissolved, via psychedelics or otherwise.