I visit many guitar forums and I've never actually seen this question posted before: how do you define 'improvisation'? To me, it's always been something like 'creating a new guitar part on the fly'. That can mean a chordal rhythm part, or a melodic solo, or adding splashes of appropriate color to a piece of music ... it has always meant 'invention'; it has always implied to me that the player is assembling the part for the first and last time ever.
What's yours?
Kirk