What is a loaded firearm?
A firearm that has ammunition loaded into it.
IMPROVING ANSWER: If you say "a gun is loaded when there is ammo
inside it" you are using common sense. If you are asking the
question about gun safety and safe gun handling, that's totally
accurate. An "unloaded" gun means one that is fully unloaded-- no
ammo in any part of it, even the magazine.
But from a legal standpoint, if you are carrying a gun in a way
that is only legal if the gun is not loaded, watch out for what
your state's legal definition of a "loaded" gun is. Not all state
and local laws are written with common sense.
Sometimes, in some jurisdictions, the law might use the phrase
"loaded firearm" to mean a gun that is NOT actually loaded, but for
which there is ammunition within easy reach, if the gun and the
ammo are not in cases or boxes. And in some jurisdictions, a gun
that has rounds in the magazine but none in the chamber might or
might not be "loaded" in the eyes of the law. So really, it depends
on what state you're in and what the circumstances are.