Dissonance is the simultaneous sounding of two or more tones which, because of beats, seems unpleasant or tense to the human ear; it is the converse of consonance. Mathematical definitions of consonance, and by association dissonance, go back to Pythagoras in the fifth century BC. Which musical intervals are considered dissonant is largely a matter of cultural and historical relativism. For instance, some intervals regarded as dissonant in the Medieval period were found to be consonant during the Renaissance. Also, the way a dissonance is resolved (and even the way it is orchestrated) can decrease or intensify how harsh it sounds. Intervals commonly considered dissonant in tonal music are the major and minor seconds and sevenths. Consonance and dissonance are in relation and, as such, are relative terms.
A dissonant chord, or discord, is a chord that sounds incomplete until it resolves itself on a harmonious chord.
See also suspension.