triad: Meaning and Definition of

tri•ad

1. a group of three, esp. of three closely related persons or things.
1. an element, atom, or group having a valence of three. Cf. monad (def. 2), dyad (def. 3).
2. a group of three closely related compounds or elements, as isomers or halides.
3. 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.) the three categories of strategic-nuclear-weapons delivery systems: bombers, land-based missiles, and missile-firing submarines.