So, Third Stream, that quicksandish place where jazz meets classical orchestral composition and both get sucked into the pit.
Generally I find the crossovers where a composer is trying to shoehorn in a bit of jazz are less successful - Gunther Schuller's stuff, Martinu, Stravinsky, "Actions For Free Jazz Orchestra" etc. but I think there is lots of interesting and unique music that could fit under this heading.
Here are some of the 'Third Stream' things I like as a kick off; what else should I be looking out for?
Charles Mingus "The Black Saint And The Sinner Lady":
Stan Kenton conducting The Los Angeles Neophonic Orchestra in Russ Garcia's "Adventure In Emotion":
Bob Graettinger's compositions played by the Stan Kenton band:
Jaroslav Jezek's Piano Concerto:
Terje Rypdal's crossover efforts - can't find a video of "Whenever I Seem To Be Far Away" which may be the best example, so here's something from the same era:

Here are some of the 'Third Stream' things I like as a kick off; what else should I be looking out for?
Charles Mingus "The Black Saint And The Sinner Lady":
Stan Kenton conducting The Los Angeles Neophonic Orchestra in Russ Garcia's "Adventure In Emotion":
Bob Graettinger's compositions played by the Stan Kenton band:
Jaroslav Jezek's Piano Concerto:
Terje Rypdal's crossover efforts - can't find a video of "Whenever I Seem To Be Far Away" which may be the best example, so here's something from the same era:
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