Since then, the definition has been broadened to view third stream as a process rather than a style, in which the music is primarily improvised and becomes a deeply personal vehicle for soloists or collaborators.
This according to “Third stream and the importance of the ear: A position paper in narrative form” by Ran Blake (College music symposium XXI/2 [fall 1981] pp. 139–46).
Today would have been Schuller’s 90th birthday! Above, delivering the Cleveland Institute of Music’s commencement address in May 2015 (a video of the address is here); below, conducting his Monk, Bunk, and vice versa in 1989.