In music, dissonance denotes a variety of intervals that are not consonant.
A dissonant interval may be melodic or harmonic and while all melodically dissonant intervals are also harmonically dissonant, the converse is not true. These intervals are thus classified because typically dissonance needs to be resolved to a consonance. Melodic dissonance usually resolves by stepwise motion in the opposite direction of the leap. Harmonic dissonance is resolved differently depending on syntax and the specific interval though most often a dissonant melodic interval will move by contrary or oblique motion to a consonant interval though it is possible for harmonic dissonance to resolve by other motion.
Dissonant melodic intervals vary by style but in Baroque harmony they are the:
tritone (augmented fourth or diminished fifth)
major and minor seventh
augmented second and all other augmented and diminished intervals
harmonically dissonant intervals include the:
tritone
major and minor second
major and minor seventh
augmented and diminished prime or unison and all other augmented and diminished intervals.
The harmonic perfect fourth is also considered to be conditionally dissonant, though this is well disputed among theorists. A harmonic perfect fourth is often considered consonant when it functions as an inverted perfect fifth but otherwise is often dissonant as the interval suggests motion to a major or minor third.
It may do well to note that with the exception of the perfect fourth, all inversions of dissonant intervals are dissonant be they melodic or harmonic. Similarly all intervals and their inversions not mentioned as dissonant are consonant and do not require resolution. While the understanding of dissonance varies with the style and period of music discussed, the above intervals may be considered an exhaustive list of dissonances in Baroque and classical styles.