If U.N. is the acronym of United Nations, then United Nations is the what of U.N.? Is there an opposite of acronym?
Merriam-Webster defines acronym as "a word formed from the initial letter or letters of each of the successive parts or major parts of a compound term (as anzac, radar, snafu)."
Strictly speaking, U.N. is an initialism:
initialism, noun: An abbreviation consisting of the first letter or letters of words in a phrase (for example, IRS for Internal Revenue Service), syllables or components of a word (TNT for trinitrotoluene), or a combination of words and syllables (ESP for extrasensory perception) and pronounced by spelling out the letters one by one rather than as a solid word. [AHED]
So what the opposite of an initialism is depends on what it was formed from in the first place.
Since U.N. was formed from a name, saying "United Nations is the full name of the U.N." is probably the proper thing to do.
The definition for backronym in the Oxford US Dictionary online sheds some light on the antonym for acronym:
a fanciful expansion of an existing acronym or word, such as “port out, starboard home” for posh.
Notice that it is "a fanciful expansion". This seems to indicate that the antonym for acronym is expansion, based on backronym's definition in the Oxford US Dictionary online as a fanciful antonym of acronym.
The most logical, symmetric term would be bathonym.
I don't know if there is a "real" (i.e. officially recognized by dictionaries or English language societies) word for this concept, but we can generate a logical and consistent one from the word acronym itself.
(Also, who cares if words are "real" or not? English has tons of irregularities and new really stupid words get added to the English dictionary all the time. It also has a lot of arbitrary disallowals of natural and logical words that are composed or roots and prefixes and suffixes for no good reason, like arrayify "to make into an array", which should have entered into English by now since it is used by programmers all over and programming is a widespread thing, and the criteria for adding words as "real" words is based on widespread usage, not whatever it makes sense. Logic and consistency over arbitrary vetoing!)
Ok, enough commentary! You can judge how sensible/logical this proposal is for yourself!
Acro- is a Greek prefix which means high (loosely, superficial, much the same concept as when we say "give me the 10,000 ft digest, which refers literally to height as a standin-for superficiality). (E.g. acropolis - literally "high city, Acrophobia - fear of heights Acrobatics - "high walker")
And nym means word/name.
So acronym means "a high level word/superficial word."
So what would be the opposite? A word that "exposes the depths and details of a word or phrase". "deep-nym".
And so we can pick the Greek prefix meaning deep, since acro is also a Greek prefix. (I hope the reader can take it as a given that consistently using the Greek is superior to arbitrarily choosing a different language like Latin)
Batho-, bathy- https://wordinfo.info/unit/287
We should choose batho- since acro also ends in o, and there is no reason to be arbitrarily inconsistent. Plus, it's also a freebie that it's easier to rember and rolls right off the tongue anyway.
So we have "bathonym"
Consistent, symmetric, beautiful.
Alternative candidates ruled out
Note: there are other Greek prefixes roughly meaning bottom, such as sub, infra, or hypo, but we should choose one that doesn't have a direct counterpart prefix already, and one that has the closest opposite meaning to acro.
Sub- The counterpart would be super. (Superliminal / subliminal) We don't call it supernym, we call it acronym. So we eliminate sub.
Hypo- (Hypertension / hypotension) Same for hypo, which has hyper, and we don't call it hypernym.
Infra- (Infrared / ultraviolet) Same with infra, which would be ultra. And it's not called ultranym either.
Batho- means deep and acro means high. Batho- is the most fitting choice by elimination and by being in the same dimension/concept as acro, namely, altitude. Batho- and acro+l- are both part of the same spectrum/dimension/concept, namely, altitude.
Edit: I guess there is also the prefix bentho-. Maybe that's better because it avoids confusion in modern English with "bath/bathing/washing" https://wordinfo.info/results?searchString=deep