I’d call it free improvisation. There was a lively discussion on this a while back over on Basschat, but it is a movement with quite a long history. A lot of it’s best known exponents are horn players such as Lol Coxhill and Evan Parker.
Amongst guitarists, best known are people like Derek Bailey and Fred Frith in his wilder outings.
Very much an acquired taste of course and not really for anybody who likes to know where the one is, but it does have a following.
For the curious, there’s plenty to choose from over on yoochoob. Not for the musically faint of heart though; you have been warned.