At least in the West, one usually thinks of art music as revolving around the idea of "composition," but improvisation has been a significant factor in many historical developments. As mentioned in the rather disjointed drones & accompaniment article, the idea of secular part-music having an origin related to improvised accompaniment has been raised. Basso continuo involved a substantial amount of improvisation, apparently in a much more individual manner than employed in Baroque reconstructions until recently. Moreover, it has long been commonly believed that organum, and with it Western polyphony in general, arose from improvising ornamental lines to existing plainchant, and there is little reason to dispute the idea. It is even increasingly suggested that Ars Subtilior music was primarily an attempt to capture improvisational practice in writing. More recently, it was expected that classical cadenzas be improvised, and improvisational techniques were an important part of every organist's — if not every classical musician's — training, at least until the early 20th century. The divorce of improvisation from "classical music" has been relatively recent, buoyed by the greater freedom in popular music, and even then it was almost as quickly countered by explicit improvisational elements inserted into postmodern compositional styles.
Improvised ornament apparently remained a part of plainchant & polyphonic choral performance in the middle ages, and virtuoso ornament was the foundation of the Florentine Camerata and with it the new monody as embraced by Monteverdi and others. Without belaboring the point further, improvisation has played an important role in much of the "classical music" of the West, at least until the ascendance of historicism and the consequent squeeze put on interpretive space. Besides Jazz, and the contemporary "classical" compositions which intersect with it most strongly, improvisation is critical to some of the most sophisticated art music traditions around the world. Within compositional frameworks, it is integral to Carnatic performances and to Indian classical music in general. Iranian music is similarly built around a pre-composed framework with improvisational bridges. Pure improvisation was historically considered to be the highest form of Arabic classical music, even if it is dying out today. This list could continue, but one might first ask whether the term "pure improvisation" makes sense. Although today one can find free improvisations, in which any & every sound or means of production might be brought into play in some manner, these traditional improvisations operated under rather specific constraints. They fit neatly into the framework of their traditions, and much of the taught theory suggested as much what was possible in an improvisation as what was possible in a composition. Arabic classical improvisation, like Indian improvisation, operated under a modal framework, and so needed to fit the specific mode. However, Arabic classical improvisation also allowed, even demanded, frequent & unexpected modulation. It is a rather free & demanding tradition in this sense, and "the most improvisational" until Jazz.
Improvisation provides not only a unifying theme for fusion efforts, but is a critical idea for preserving interpretive space and resisting or even subsuming effects of a collection mentality, not to mention for the future of music in the context of the web and the possibility of developing a new mainstream out of the smaller world created by Internet communication. I claim that improvising is an important part of interacting with the world, a fact which should be evident to anyone who has had mishaps in his daily life, and consequently a necessary part of maintaining relevance in music. Such a need becomes more explicit as sounds are more often committed to a medium, and not ephemeral as in the past. The whole idea of improvising with pre-recorded sounds must be examined, perhaps in a way analogous to the work of disc jockeys, or as an extension of general thoughts on collage. The Internet & multimedia do provide the flexibility to manipulate entire "chunks" of content, possibly allowing a way to have more variety in composer-as-performer scenarios, while making use of a composed versus performance-centric dynamic in a sort of upside down way. On the broadest scale, such a thing could be built in interpretive or improvisational layers, so as to make it totally unclear what is new, what is old, who is doing what, and especially what is real. I think we need to do such things explicitly in order to draw attention to the entire phenomenon of framing content, of images within images, and even paradoxically as a reaction to the increasing lack of silence. I have this notion of a real "stillness" created in the seams of an interpretation of an interpretation and beyond, as it all unfolds in real time.
The above suggests, indeed, one gigantic improvisation with multiple unrelated performers, unfolding all the time, and without truly static elements. The idea of a fixed document might be flawed, or rather composition itself might be bound to misdirection. The entire vision is both already true & ridiculous, making it less something to be accomplished than something to be experienced. It also suggests even more questions regarding the space out of which contemporary art can carve itself, if indeed it is to have a separate existence at all. If everything is to some degree an improvisation — not to take a real stance on fate versus free will — then there is a transcendence available in the moment of action, and if people can be connected in their actions (whether explicitly or not), we have a form of communion. This is true regardless of attitude, in some sense, but the issue of letting people take positive feelings from the experience remains. That issue involves overcoming a growing inability to feel, and improvisation does provide one connection between a more static world of images and the animal nature of movement and doing. The whole notion allows people to be involved in the doing, and that is probably the most critical thing today. Improvisation framing composition or composition framing improvisation? The entire distinction may become artificial, as the stasis-inducing force of committing everything to a digital archive has the effect of making anything that actually happens seem more fluid. Information saturation will likely yield a heightened sense of exhilaration to true improvisation, and further marginalize exactly the pre-composed forms it seeks to preserve.