I can describe "IMHO" as an acronym for "in my humble opinion." How can I describe the reverse processes of translating the acronym into the actual phrase? That is to say, if "IMHO" is the acronym, what do we call "in my humble opinion"?
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The spelled-out form of an acronym or initialism (that is, "what it stands for") is called its expansion.
I think you might have answered your own question: definition is probably the best word, though expansion could also work (because an acronym is a type of abbreviation).