3. triad a group of three closely related compounds or elements, as isomers or halides.
4. triad the basicchord of a musical tonality, consisting of a tonic, a third, and a fifth.
Etymology: (1540–50; < L triad-, s. of trias < Gk triás. See tri -, -ad1)
Definition of 'triad'
Princeton's WordNet
1. (noun)three, 3, III, trio, threesome, tierce, leash, troika, triad, trine, trinity, ternary, ternion, triplet, tercet, terzetto, trey, deuce-ace the cardinalnumber that is the sum of one and one and one
2. (noun)trio, triad, triplet, triple a set of three similar things considered as a unit
3. (noun)trio, threesome, triad, trinity threepeople considered as a unit
4. (noun)common chord, triad a three-note major or minor chord; a note and its third and fifth tones
Definition of 'triad'
Webster Dictionary
1. (noun)triad a union of three; three objects treated as one; a ternary; a trinity; as, a triad of deities
Definitions of triad 
