jazz A type of music developed mostly by African Americans in the early part of the twentieth century that combined elements of African, popular, and European music, and that has evolved into a broad tradition encompassing many styles.
jongleur (French) Itinerant medieval musician or street entertainer.
jubilus (Latin) In CHANT, an effusive MELISMA, particularly the melisma on "-ia" in an ALLELUIA.
just intonation A system of tuning NOTES in the SCALE, common in the RENAISSANCE, in which most (but not all) thirds, sixths, perfect fourths, and perfect fifths are in perfect tune.