My 2ç as also a (very minor) terraform core, and numerous providers & modules contributor, and user (also 2015 I think): I've never heard of 'terraform stacks' before this thread, but 'root module' makes perfect sense to me:

1) without the context of where the state is/its contents, or estimating based on the resources/style/what's variable vs. hardcoded, a 'stack' (if you like) is indistinguishable from a module that's to be used in someone else's 'stack';

2) `path.root`