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.