Pronunciation: (trī'ad, -ud), [key]
1. a group of three, esp. of three closely related persons or things.
a. an element, atom, or group having a valence of three. Cf. monad (def. 2), dyad (def. 3).
b. a group of three closely related compounds or elements, as isomers or halides.
3. Music.a chord of three tones, esp. one consisting of a given tone with its major or minor third and its perfect, augmented, or diminished fifth.
4. (cap.) Mil.the three categories of strategic-nuclear-weapons delivery systems: bombers, land-based missiles, and missile-firing submarines.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.