[trahy-ad, -uhd]
a group of three, especially of three closely related persons or things.
an element, atom, or group having a valence of three. Compare monad ( def 2 ), dyad ( def 3 ).
a group of three closely related compounds or elements, as isomers or halides.
Music. a chord of three tones, especially one consisting of a given tone with its major or minor third and its perfect, augmented, or diminished fifth.
(initial capital letter) Military. the three categories of strategic-nuclear-weapons delivery systems: bombers, land-based missiles, and missile-firing submarines.

1540–50; < Latin triad- (stem of trias) < Greek triás See tri-, -ad1

triadic, adjective
triadism, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
triad (ˈtraɪæd)
1.  a group of three; trio
2.  chem an atom, element, group, or ion that has a valency of three
3.  music a three-note chord consisting of a note and the third and fifth above it
4.  an aphoristic literary form used in medieval Welsh and Irish literature
5.  the US strategic nuclear force, consisting of intercontinental ballistic missiles, submarine-launched ballistic missiles, and bombers
[C16: from Late Latin trias, from Greek; related to Greek treis three]

Triad (ˈtraɪæd)
any of several Chinese secret societies, esp one involved in criminal activities, such as drug trafficking

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1546, "group or set of three," from L.L. trias (gen. triadis), from Gk. trias (gen. triados), from treis "three" (see three). Musical sense of "chord of three notes" is from 1801.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

triad tri·ad (trī'ād', -əd)

  1. A collection of three things or symptoms having something in common.

  2. The transverse tubule, and the terminal cisternae on each side of it, in a skeletal muscle fiber.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Encyclopedia Britannica


in chemistry, any of several sets of three chemically similar elements, the atomic weight of one of which is approximately equal to the mean of the atomic weights of the other two. Such triads-including chlorine-bromine-iodine, calcium-strontium-barium, and sulfur-selenium-tellurium-were noted by the German chemist J.W. Dobereiner between 1817 and 1829. The triad was the earliest atomic-weight classification of the elements

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
All of these paradoxes form a triad that is the way of everything.
The second triad harks back to the central principle of scarcity.
The major interval contained within the scale generates an implied major triad built on the fourth degree of the scale.
It would be the movie's biggest surprise if he weren't the brains behind the triad.
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