What is Jazz Harmony?
Before reading my brief summary of the fundamentals of Jazz Harmony, I have provided a video that I would recommend you watch until 10:35 as a primer:
A brief synthesis of an agreed-upon definition of jazz harmony:
Jazz harmony is the theory and practice of how chords are used in jazz music. A chord is a harmonic set of pitches that consists of multiple notes that are heard as if sounding simultaneously. Jazz is similar to other practices in the tradition of Western harmony. Chord progressions and the use of major and minor scales as a basis for chordal construction are commonly found in both (Western harmony and Jazz specifically). In jazz, chords are often arranged vertically in major or minor thirds, although stacked fourths are also often found. Furthermore, jazz includes the addition of tensions, and intervals such as 9ths, 11ths, and 13ths to chords. Unlike classical music, which favors the use of triads (3-note chords), jazz harmony tends towards the use of seventh chords (“tetrachords” or chords consisting of 4 notes with possible extensions comprising an “upper structure”) as the basic harmonic unit.