Well, Compound adverbs was my first response, too. But I confess that the root of my question is actually a friend, who is seeking a particularly perplexing crossword answer, and I guess compound adverb doesn't fit the bill. Given that English has a startling number of words which define very precise things ("We needed a word for that?") I thought I'd investigate.

New information suggests that the answer will likely be a compound itself - _ _R_E WORDS. (I'm guessing at the 'words' part, the crossword puzzle complete currently has _ _ R _ E _ O _ D S. Surely, now that I'm given that, something clever rather than an obscure definition.

Zmjezhd, I submit that while most of your points are perfectly valid, in terms of 'why this and not that', our heavy reliance on prepositions in English make 'into' generally mean something quite different from 'in to'.

Thanks, everyone, for your help! If someone does come up with that very precise definition, I'd still quite like to know it!