Is there a word for the definition of an acronym?
Solution 1:
The spelled-out form of an acronym or initialism (that is, "what it stands for") is called its expansion.
http://en.wikipedia.org/wiki/Acronym_and_initialism
Solution 2:
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).