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"?
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).