In music, consonance and dissonance are categorizations of simultaneous or successive sounds. Within the Western tradition, some listeners associate consonance with sweetness, pleasantness, and acceptability, and dissonance with harshness, unpleasantness, or unacceptability, although there is broad acknowledgement that this depends also on familiarity and musical expertise. The terms form a structural dichotomy in which they define each other by mutual exclusion: a consonance is what is not dissonant, and a dissonance is what is not consonant. However, a finer consideration shows that the distinction forms a gradation, from the most consonant to the most dissonant. In casual discourse, as German composer and music theorist Paul Hindemith stressed,

The term sonance has been proposed to encompass or refer indistinctly to the terms consonance and dissonance.

The opposition between consonance and dissonance can be made in different contexts:

In both cases, the distinction mainly concerns simultaneous sounds; if successive sounds are considered, their consonance or dissonance depends on the memorial retention of the first sound while the second sound (or pitch) is heard. For this reason, consonance and dissonance have been considered particularly in the case of Western polyphonic music, and the present article is concerned mainly with this case. Most historical definitions of consonance and dissonance since about the 16th century have stressed their pleasant / unpleasant, or agreeable / disagreeable character. This may be justifiable in a psychophysiological context, but much less in a musical context properly speaking: Dissonances often play a decisive role in making music pleasant, even in a generally consonant context – which is one of the reasons why the musical definition of consonance/dissonance cannot match the psychophysiologic definition. In addition, the oppositions pleasant/unpleasant or agreeable/disagreeable evidence a confusion between the concepts of "dissonance" and of "noise". (See also Noise in music and Noise music.)

While consonance and dissonance exist only between sounds and therefore necessarily describe intervals (or chords), such as the perfect intervals, which are often viewed as consonant (e.g., the unison and octave). Occidental music theory often considers that, in a dissonant chord, one of the tones alone is in itself deemed to be the dissonance: It is this tone in particular that needs "resolution" through a specific voice leading procedure. For example, in the key of C Major, if F is produced as part of the dominant seventh chord (G⁷, which consists of the pitches G, B, D and F), it is deemed to be "dissonant" and it normally resolves to E during a cadence, with the G⁷ chord changing to a C Major chord.

Scientific definitions have been variously based on experience, frequency, and both physical and psychological considerations. These include:

A stable tone combination is a consonance; consonances are points of arrival, rest, and resolution.

— Roger Kamien