You know what I mean though.
Well, no... not really. The problem is that "impromptu" is used inconsistently, both within the general magic community and this forum. It's use as a marketing term muddies the waters even further.
Strictly speaking, "impromptu" means "made or done without previous preparation"; however, it is also used to mean "with little
or no preparation" and "with the appearance
of having required little or no preparation."
This is complicated even further by the introduction of the distinction between magic with borrowed items, magic with ordinary objects, and magic with whatever happens to be on hand.
Based on the definitions you choose to use, you could argue that there is, for example, no impromptu card magic because you need a deck of cards, thus requiring some sort of "preparation." Or you could argue that carryign around a deck of cards simply constitutes "little preparation," which is okay. Or you could say that you're only doing impromptu card magic if it is with a deck of cards somebody hands you. But what if I happen to always carry around a deck of cards set in the Breakthrough Card System? Is that "little preparation"? What if I always carry a TT?
You can simply say that impromptu magic is magic that you can do on the spur of the moment if someone asks... regardless of how much preparation is involved or whether or not it requires a gimmick. That makes "impromptu" very subjective, as it is purely dependent on what you are capable of and what you carry around with you.
As you can see, it can get a little complicated unless you specifically define your terms.