Etymology
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triad (n.)
1540s, "group or set of three," from Late Latin trias (genitive triadis), from Greek trias (genitive triados) "a triad, the number three," from treis "three" (see three). Musical sense of "chord of three notes" is from 1801. Related: Triadic.
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Triassic (adj.)
1841, from German, coined 1841 by German geologist Friedrich August von Alberti (1795-1878), from Trias "period preceding the Jurassic," from Greek trias "triad, the number three" (see triad). So called because it is divisible (in Germany) into three groups.
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word-forming element of Greek origin appended to nouns and denoting collective numerals (triad, Olympiad) and fem. patronymics (Dryad, Naiad, also, in plural, Pleiades, Hyades), thence also plant family names; from Greek -as (genitive -ados), fem. suffix equivalent to -is.

From its use in Iliad (literally "of Ilion," that is, "Troy;" from Ilias poiesis or oidos "poem of Ilion," the accompanying noun being feminine, hence the termination) it has formed titles of poems in imitation of it (Columbiad, Dunciad).

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trinity (n.)

early 13c., "the Father, Son and Holy Spirit," constituting one God in prevailing Christian doctrine, from Old French trinite "Holy Trinity" (11c.), from Late Latin trinitatem (nominative trinitas) "Trinity, triad" (Tertullian), from Latin trinus "threefold, triple," from plural of trini "three at a time, threefold," related to tres (neuter tria) "three" (see three).

The Latin word was widely borrowed in European languages with the spread of Christianity (Irish trionnoid, Welsh trindod, German trinität). Old English used þrines as a loan-translation of Latin trinitas. Related: Trinitarian.

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