The only thing that's actually treated as a firearm, generally, is firearms. If it doesn't rely on burning gases, it's not a firearm.
That said, there's any number of situations where folks will tell you otherwise. A friend in HS had his paintball gun confiscated by a LEO during a traffic stop "because he was under 18." We do have laws about imitation firearms - if your CO2 pistol looks convincing, you could get jammed up on that in certain circumstances, or if you're transporting on a plane, you have to check it like a real firearm.
But with respect to licenses, etc. a CO2 pistol is an airgun, and airguns aren't firearms.