1) Atonalism 2) Dissonance 1) Atonalism 2) Musical notation 3) The absence of a key 
Atonality in its broadest sense is music that lacks a tonal center, or key. Atonality, in this sense, usually describes compositions written from about 1908 to the present day where a hierarchy of pitches focusing on a single, central tone is not used, and the notes of the chromatic scale function independently of one another {harv|Kennedy|1994}. ...
in music, the absence of functional harmony as a primary structural element. The reemergence of purely melodic-rhythmic forces as major determinants ... [7 related articles]
Strictly a description of music that rejects traditional tonality, but it is commonly used to describe music using unfamiliar harmony.
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
Total abandonment of tonality. Atonal music moves from one level of dissonance to another.
atonalism noun the absence of a key; alternative to the diatonic system
Music that has no sense of tonality and no obvious key. Atonal music uses the notes of the chromatic scale and, depending on the system employed, uses all twelve pitch classes in hierarchies other than triadic harmony. This means that there is no pull towards any particular tonic note. Arnold Schoenberg was one of the first composers to explore ato...
absence of key or central tonality.
the absence of key or tonal center. · an atonal principle or style of composition.