triad

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A group of three.
• noun Music A chord of three tones, especially one built on a given root tone plus a major or minor third and a perfect fifth.
• noun A section of a Pindaric ode consisting of the strophe, antistrophe, and epode.

from The Century Dictionary.

• noun A union or conjunction of three; a group or class of three persons or things closely related; a trinity.
• noun In chem., an element or radical which will combine with three atoms of a monad element or radical; a trivalent element or radical.
• noun In music, a chord of three tones, including a given tone with its major or minor third and its perfect, augmented, or diminished fifth. A triad is named from the given tone or root: as, triad of G; dominant triad. See chord, 4. Also trias.
• noun In Welsh lit., a form of composition characterized by the arrangement of the contents in groups of three.
• noun In mythology, an intimate association of three kindred or correlated deities, sometimes considered as having the relationship of father, mother, and child, and forming a characteristic conception in some religious systems, as that of ancient Egypt.
• noun In morphology, a tertiary unit of organization resulting; from integration of an aggregate of dyads. See dyad, 3.
• noun An indeterminate product of three vectors.
• noun plural See the Triad Society.

from the GNU version of the Collaborative International Dictionary of English.

• noun A union of three; three objects treated as one; a ternary; a trinity.
• noun A chord of three notes.
• noun The common chord, consisting of a tone with its third and fifth, with or without the octave.
• noun (Chem.) An element or radical whose valence is three.
• noun poetical histories, in which the facts recorded are grouped by threes, three things or circumstances of a kind being mentioned together.
• noun See Trimurti.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A grouping of three.
• noun music A chord consisting of a root tone, the tone two degrees higher, and the tone four degrees higher in a given scale.
• noun electronics on a CRT display, a group of three neighbouring phosphor dots, coloured green, red, and blue.
• noun A branch of a Chinese underground criminal society, mostly based in Hong Kong.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun three people considered as a unit
• noun a three-note major or minor chord; a note and its third and fifth tones
• noun a set of three similar things considered as a unit
• noun the cardinal number that is the sum of one and one and one