In music, a consonance (Latin con-, "with" + sonare, "to sound") is a harmony, chord, or interval considered stable (at rest), as opposed to a dissonance (Latin dis-, "apart" + sonare, "to sound"), which is considered unstable (or temporary, transitional). In more general usage, a consonance is a combination of notes that sound pleasant to most people when played at the same time; dissonance is a combination of notes that sound harsh or unpleasant to most people.
Blend/fusion: perception of unity or tonal fusion between two notes (Stumpf)
Frequency ratios: with ratios of lower simple numbers being more consonant than those that are higher (Pythagoras). Many of these definitions do not require exact integer tunings, only approximation.
Coincidence of partials: with consonance being a greater coincidence of partials (called harmonics or overtones when occurring in harmonic timbres) (Helmholtz, 1877/1954). By this definition, consonance is dependent not only on the width of the interval between two notes (i.e., the musical tuning), but also on the combined spectral distribution and thus sound quality (i.e., the timbre) of the notes (see the entry under critical band). Thus, a note and the note one octave higher are highly consonant because the partials of the higher note are also partials of the lower note. Although Helmholtz's work focused almost exclusively on harmonic timbres and tunings, subsequent work has generalized his findings to embrace non-harmonic tunings and timbres.
Fusion or pattern matching: fundamentals may be perceived through pattern matching of the separately analyzed partials to a best-fit exact-harmonic template (Gerson & Goldstein, 1978) or the best-fit subharmonic (Terhardt, 1974). Harmonics may be perceptually fused into one entity â€” consonances being those that include:
unisons and octaves
perfect fourthsa and perfect fifths
major thirds and minor sixths
minor thirds and major sixths
"Continuity: consonances are continuous and dissonances are intermittent in sensation, determined by coincidence of partials." (Helmholtz) Wikipedia, Consonance and Dissonance