It seems self-evident to me, but in the heat of a Scrabble game (no surprise), my opponent claimed that "sh" was a word. I think it's a diphthong, but the printed dictionary definition of "word" didn't resolve the issue.
I know that Scrabble-related questions on this site are generally regarded as infra dig, but the event really did cause me to wonder what truly qualifies as a "word" and what doesn't.
[I also understand that this question sits just atop the divide between meaning and orthography, but hey, whtvr.]