Definition of: triad

(trī′ad) noun
1. A group of three persons or things.
2. Music A chord of three tones or notes; often the common chord, consisting of a fundamental tone with its third and fifth higher. A major triad has a major third and a perfect fifth; a minor triad has a minor third and a perfect fifth.
3. Chem. a A trivalent atom or radical. b One of a group of three elements having similar chemical properties, as chlorine, bromine, and iodine. [<L trias, -adis <Gk. trias, -ados <treis three]
—tri·ad′ic adjective & noun