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n. Music The absence of a tonal center and of harmonies derived from a diatonic scale corresponding to such a center; lack of tonality.

n. Music A style of composition that lacks such a tonal center and its derived harmonies.

n. A style of music that is written without a key.

n. A passage written without a key.

n. the absence of a key; alternative to the diatonic system.

n. the absence of a key; alternative to the diatonic system

"(Which, interestingly, would mean that the reaction against atonality is less about the intrinsic properties of tonal harmonies and more about it’s ability to create the illusion of rhythmic symmetry.)"

musical notation   

diatonic    tonality   

atonalism