triad

[trahy-ad, -uh d]
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noun
  1. a group of three, especially of three closely related persons or things.
  2. Chemistry.
    1. an element, atom, or group having a valence of three.Compare monad(def 2), dyad(def 3).
    2. a group of three closely related compounds or elements, as isomers or halides.
  3. 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.
  4. (initial capital letter) Military. the three categories of strategic-nuclear-weapons delivery systems: bombers, land-based missiles, and missile-firing submarines.

Origin of triad

1540–50; < Latin triad- (stem of trias) < Greek triás See tri-, -ad1
Related formstri·ad·ic, adjectivetri·ad·ism, noun
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018


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British Dictionary definitions for triad

triad

noun
  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
Derived Formstriadic, adjectivetriadism, noun

Word Origin for triad

C16: from Late Latin trias, from Greek; related to Greek treis three

Triad

noun
  1. any of several Chinese secret societies, esp one involved in criminal activities, such as drug trafficking
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for triad
n.

1540s, "group or set of three," from Late Latin trias (genitive triadis), from Greek trias (genitive triados), from treis "three" (see three). Musical sense of "chord of three notes" is from 1801.

Online Etymology Dictionary, © 2010 Douglas Harper

triad in Medicine

triad

[trīăd′, -əd]
n.
  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.