impromptu: adj. & adv. extempore, unrehearsed. (The Concise Oxford Dictionary)
extempore: adj. & adv. 1 without preparation. 2 offhand. [ L ex tempore on the spur of the moment... ] (Oxford)
impromptu: [F.] Title for short pieces by… Chopin and others, suggesting an offhand or extemporized style. In style and form, however, these are not markedly different from some other types of character pieces. (Harvard Concise Dictionary of Music)
Here’s a markedly different, extemporized rendition of Chopin’s Fantasie-Impromptu. It’ll get your toes tapping in a way that the original never did. My first thought – this is much easier than Chopin’s written version – but I’m not so sure. Rhythmically, yes, but there’s a whole lot of jumping going on here.