For my tuppence, "Impromptu" is a word bandied around too frequently within magic, and dealer's advertisements, to hold any kind of constant meaning.
In my head, I separate this into several categories:
Apparently impromptu - for example, a card revelation where you've had to prepare the environment beforehand - stuffing a duplicate six of spades in a loaf of bread, run up a telephone pole and covered in bees, which you "just happen" to be passing. For my money, a pre-stacked deck would fit here.
Impromptu - a set of "magic" tools you can carry around with you, to unleash the moment when the time is right. Possibly even an (unstacked) deck of cards, TT, loops etc. would fall into this category. The term "impromptu" I do not think is technically accurate here, but I think it fits it's common usage. Much like the term "Mentalism". in fact, which of course has nothing to do with reading minds.
"Jazz" or improvised magic fits into the above category - having the basic tools available to raise hell and all their little wizards, without necessarily a routine in mind, or not a fully fleshed out routine at least.
Unprepared Hardcore. Possibly no routines, certainly no props. Psychological forces, and sleight of hand with borrowed objects. As this morning, in fact, when a fortuitous bowl of chestnuts and some coin moves came into unison
There are probably more definitions, and these are only my opinions, which rarely mean anything
