Note that "loaded / unloaded" may be specified by law.
Ohio:
(this applies to firearms in a vehicle, so (i)(ii) don't appli to the term in general)
(5)(a) "Unloaded" means, with respect to a firearm other than a firearm described in division (K)(6) of this section, that no ammunition is in the firearm in question, no magazine or speed loader containing ammunition is inserted into the firearm in question, and one of the following applies:
(i) There is no ammunition in a magazine or speed loader that is in the vehicle in question and that may be used with the firearm in question.
(ii) Any magazine or speed loader that contains ammunition and that may be used with the firearm in question is stored in a compartment within the vehicle in question that cannot be accessed without leaving the vehicle or is stored in a container that provides complete and separate enclosure.