Triad \Tri"ad\, n. [L. trias, -adis, Gr. ?, ?, fr.?, ?, three: cf. F. triade. See Three, and cf. Trias, Trio.]

1. A union of three; three objects treated as one; a ternary; a trinity; as, a triad of deities.

2. (Mus.)
   (a) A chord of three notes.
   (b) The common chord, consisting of a tone with its third and fifth, with or without the octave.

3. (Chem.) An element or radical whose valence is three.

Triads of the Welsh bards, poetical histories, in which the facts recorded are grouped by threes, three things or circumstances of a kind being mentioned together.

Hindu triad. See Trimurti.

Source: Webster's Revised Unabridged Dictionary (1913)