One should consider this a general reference question. However:
Why would we have a tag "single-word requests" if there was a better term for that? Technically, there's no such thing as a monoword, though polyword is well known, mostly as an adjective.
If you chose to be informal:
- monoword. Being of few words, being quite cold in the reply.
Wikipedia mentions word sentences, though I think that's more of an academic term. (The link under that term contradicts, in a way, the very sentence from where it is drawn!)
a set of words that is complete in itself, typically containing a subject and predicate, conveying a statement, question, exclamation, or command, and consisting of a main clause and sometimes one or more subordinate clauses.
A "set" can consist of any number of words, including one, or even none.
Further to that, everything else is qualified with "typical" but not necessarily.
"Yes." is a sentence. QED.
By the way, "Yes, I am." is not monosyllabic.