Etymology
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tripod (n.)
c. 1600, "three-legged vessel," c. 1600, from Latin tripus (genitive tripodis), from Greek tripous (genitive tripodos) "a three-legged stool or table," noun use of adjective meaning "three-footed," from tri- "three" (see tri-) + pous (genitive podos) "foot" (from PIE root *ped- "foot"). Related: Tripodal.
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triglyceride (n.)
1860, malformed from tri- + glyceride. So called for the three radicals which replace the three hydrogen atoms.
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triceps (n.)
the great extensor muscle, 1704, from Latin triceps "three-headed," from tri- "three" (see tri-) + -ceps, from caput "head" (from PIE root *kaput- "head"). So called because the muscle has three origins.
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triangle (n.)
late 14c., from Old French triangle (13c.), from Latin triangulum "triangle," noun use of neuter of adjective triangulus "three-cornered, having three angles," from tri- "three" (see tri-) + angulus "corner, angle" (see angle (n.)).
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trimester (n.)

1821, "period of three months," from French trimestre (early 17c.), from Latin trimestris "of three months," from tri- "three" (see tri-) + mensis "month" (see moon (n.)). Specific obstetrics sense is attested from 1900. Related: Trimestrial.

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triplet (n.)

1650s, "three successive lines of poetry," from triple; perhaps patterned on couplet. Extended to a set of three of anything by 1733, and to three children at the same birth by 1787 (another word for this was trin, 1831, on the model of twin). Musical meaning "three notes played in the time of two" is from 1801.

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tricentennial (adj.)

also tri-centennial, "comprising three hundred years; including or relating to an interval of three hundred years," 1818; as a noun, "day observed as a festival in commemoration of something that happened three hundred years before," 1872. See tri- + centennial.

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trireme (n.)
"ancient ship with three rows of oars," c. 1600, from Latin triremis, from tri- "three" (see tri-) + remus "oar" (from PIE root *ere- "to row").
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trinomial (adj.)
1670s, "having three names," from tri- + second element from binomial. In mathematics, "consisting of three terms" (1704).
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triptych (n.)
"three-part altar-piece carvings or pictures hinged together," 1849, based on Italian triptica, from tri- "three" on model of diptych.
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