From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
[1913 Webster]
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.
[1913 Webster]
3. (Chem.) An element or radical whose valence is three.
[1913 Webster]
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.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
triad
n 1: the cardinal number that is the sum of one and one and one
[syn: three, 3, III, trio, threesome, tierce,
leash, troika, triad, trine, trinity, ternary,
ternion, triplet, tercet, terzetto, trey, {deuce-
ace}]
2: a set of three similar things considered as a unit [syn:
trio, triad, triplet, triple]
3: three people considered as a unit [syn: trio, threesome,
triad, trinity]
4: a three-note major or minor chord; a note and its third and
fifth tones [syn: common chord, triad]