1950; see atonal + -ity.

Strictly a description of music that rejects traditional tonality, but it is commonly used to describe music using unfamiliar harmony.

Music without tonality, or music that is centered around no central key or scale. The term was first used to describe certain compositions by Arnold Schoenberg, Anton von Webern, and Alban Berg. Schoe [..]

  In music, rejection of traditional harmonic elements including the diatonic scale  and an obvious tonal center, such as in the music of Austrian-born Arnold Shönberg (1874-1951).

  Music with an absence of tonality. Traditional tonal structures are intentionally ignored or abandoned in this contemporary form of music. 

(n) the absence of a key; alternative to the diatonic system

absence of key or central tonality.

Avoidance of centering around a specified note or key area.

[0,1] an apparent lack of key.

(uncountable,music) A style of music that is written without a key. (countable,music) A passage written without a key.