"There's no such thing as improvisation," the Grateful Dead's Jerry Garcia once said. "There's only composition. Only you do it quickly; you're composing on the spot."
This seems like a suspiciously semantic denial, but, if improvisational music does exist, what is it and what defines it? When a conductor interprets "St. Matthew Passion," what does that have in common with B.B. King playing whatever feels right over a 12-bar blues, or Ravi Shankar exploring the melodic possibilities of a raga, or John Coltrane working "My Favorite Things" until it explodes?
Clearly, the scope of improvisation in music is vast. Perhaps it is enough just to recognize that musicians have forever experimented with time and tone, crossing boundaries where it felt right to and, well, played with music as they play music.