1680s, from French trillion, from Italian trilione; see tri- + million. In the U.S., the fourth power of a thousand (one thousand billion, 1 followed by 12 zeroes); in Great Britain, the third power of a million (one million billion, 1 followed by 18 zeroes), which is the original sense. Compare billion.
1820, from trillion + -th (1).
trillium (n.)
1768, from Modern Latin trillium (Linnaeus, 1753), from Latin tri- "three" (see three). So called for its leaves and flower segments.
trilobite (n.)
extinct marine arthropod, 1820, from Modern Latin Trilobites (Walch, 1771), from Greek tri- "three" (see three) + lobos "lobe" (see lobe); so called because its body is divided into three lobes.
trilogy (n.)
series of three related works, 1660s, from Greek trilogia "series of three related tragedies performed at Athens at the festival of Dionysus," from tri- "three" (see three) + logos "story" (see Logos).
trim (n.)
"state of being prepared," 1580s, nautical jargon, "fit for sailing," from trim (v.). From 1570s as "ornament, decoration;" the meaning "visible woodwork of a house" is recorded from 1884; sense of "ornamental additions to an automobile" is from 1922. Slang meaning "a woman regarded as a sex object" is attested from 1955, American English.
trim (adj.)
c. 1500, "neatly or smartly dressed," probably ultimately from trim (v.) or from related Old English trum "firm, fixed, secure, strong, sound, vigorous, active." Related: Trimly; trimness.
trim (v.)
mid-15c., probably from Old English trymian, trymman "strengthen, fortify, confirm; comfort; incite; set in order, arrange, prepare, make ready; become strong," from trum "strong, stable," from Proto-Germanic *trum-, from PIE *dru-mo-, suffixed form of root *deru- "be firm, solid, steadfast." Examples in Middle English are wanting.

Original sense is preserved in nautical phrase in fighting trim (see trim (n.)); where the verb meant "distribute the load of a ship so she floats on an even keel" (1570s). Meaning "make neat by cutting" is first recorded 1520s; that of "decorate, adorn" is from 1540s. Sense of "reduce" is attested from 1966.
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 1916. Related: Trimestrial.
trimeter (n.)
"a verse of three metrical feet," 1560s, from Latin trimetrus, from Greek trimetros "having three measures," from tri- "three" (see tri-) + metron "a measure" (from PIE root *me- (2) "to measure"). Related: Trimetrical.
trimmer (n.)
1550s, "one who trims," agent noun from trim (v.). Meaning "one who changes opinions, actions, etc. to suit circumstances" is from 1680s, from the verb in the nautical sense of "adjust the balance of sails or yards with reference to the wind's direction" (1620s).
trimmings (n.)
"adornments, accessories, etc.," 1610s, from trim (v.).
trine (adj.)
"threefold," late 14c., from Old French trine "triple, threefold" (13c.), from Latin trinus "threefold," from tres "three" (see three).
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 rise of Christianity (Irish trionnoid, Welsh trindod, German trinität). Related: Trinitarian.
trinket (n.)
1530s, of unknown origin. Evidently a diminutive form, perhaps related to trick (n.).
trinomial (adj.)
1670s, "having three names," from tri- + second element from binomial. In mathematics, "consisting of three terms" (1704).
trio (n.)
1724, "composition for three voices," from French trio (c. 1600), from Italian trio, from tri- "three" (see three); patterned on duo. Meaning "group of three persons" is from 1789.
triolet (n.)
verse form, from French triolet, a diminutive of trio (see trio).
trip (v.)
late 14c., "tread or step lightly and nimbly, skip, dance, caper," from Old French triper "jump around, dance around, strike with the feet" (12c.), from a Germanic source (compare Middle Dutch trippen "to skip, trip, hop; to stamp, trample," Low German trippeln, Frisian tripje, Dutch trappen, Old English treppan "to tread, trample") related to trap (n.).

The senses of "to stumble" (intransitive), "strike with the foot and cause to stumble" (transitive) are from mid-15c. in English. Meaning "to release" (a catch, lever, etc.) is recorded from 1897; trip-wire is attested from 1868. Related: Tripped; tripping.
trip (n.)
"act or action of tripping" (transitive), early 14c., from trip (v.); sense of "a short journey or voyage" is from mid-15c.; the exact connection to the earlier sense is uncertain. The meaning "psychedelic drug experience" is first recorded 1959 as a noun; the verb in this sense is from 1966, from the noun.
tripartite (adj.)
"divided in three," early 15c., from Latin tripartitus "divided into three parts," from tri- "three" (see three) + partitus, past participle of partiri "to divide" (from pars "a part, piece, a share," from PIE root *pere- (2) "to grant, allot").
tripe (n.)
c. 1300, from Old French tripe "guts, intestines, entrails used as food" (13c.), of unknown origin, perhaps via Spanish tripa from Arabic therb "suet" [Klein, Barnhart]. Applied contemptuously to persons (1590s), then to anything considered worthless, foolish, or offensive (1892).
triple (adj.)
early 15c., from Old French triple or directly from Latin triplus "threefold, triple," from tri- "three" (see tri-) + -plus "-fold" (see -plus). As a noun, early 15c., "a triple sum or quantity," from the adjective. The baseball sense of "a three-base hit" is attested from 1880. Related: Triply (adv.). Triple-decker is from 1940 of sandwiches and wedding cakes, 1942 of beds.
triple (v.)
late 14c., from Medieval Latin triplare "to triple," from Latin triplus "threefold, triple" (see triple (adj.)). Related: Tripled; tripling.
triplet (n.)
1650s, "three successive lines of poetry," from triple; perhaps patterned on doublet. 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.
triplicate (v.)
"to multiply by three," 1620s, from Latin triplicatus (see triplicate (adj.)). Related: Triplicated; triplicating; triplication.
triplicate (adj.)
early 15c., "triple, threefold," from Latin triplicatus, past participle of triplicare "to triple," from tri- "three" (see tri-) + plicare "to fold" (from PIE root *plek- "to plait").
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.
both the Libyan capital and the Lebanese port city represent Greek tri- "three" (see tri-) + polis "city" (see polis). In Libya, Tripolis was the name of a Phoenician colony consisting of Oea (which grew into modern Tripoli), Leptis Magna, and Sabratha. Arabic distinguishes them as Tarabulus ash-sham ("Syrian Tripoli") and Tarabulus al-garb ("Western Tripoli").
triptych (n.)
"three-part altar-piece carvings or pictures hinged together," 1849, based on Italian triptica, from tri- "three" on model of diptych.
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").
Triscuit (n.)
proprietary name for a type of cracker, 1906, curiously from tri- + biscuit.
trisect (v.)
1690s, from tri- "three" + Latin sectus "cut," past participle of secare "to cut" (from PIE root *sek- "to cut"). Probably patterned on bisect. Related: Trisected; trisecting; trisection (1660s).
trisexual (adj.)
by 1986, from tri- + sexual.
triskaidekaphobia (n.)
"fear of the number 13," 1908, also triskaidecaphobia, from Greek treiskaideka, triskaideka "thirteen" (from treis "three" + deka "ten") + -phobia "fear."
triskelion (n.)
"figure consisting of three branches radiating from a center," 1880, earlier triskelos (1857), from Greek triskeles "three-legged," from tri- "three" (see tri-) + skelos "leg" (see scoliosis).
trismus (n.)
"lockjaw," 1690s, Modern Latin, from Greek trismos "a scream; a grinding, rasping," akin to trizein "to chirp, gnash," imitative.
trisomy (n.)
1930, from trisome (from tri- + ending from chromosome) + -y (4).
trist (adj.)
"sorrowful," early 15c., from French triste "sad, sadness" (10c.), from Latin tristis "sad, mournful, sorrowful, gloomy." Re-borrowed late 18c. (as "dull, uninteresting") as a French word in English and often spelled triste.
masc. proper name, name of a medieval hero, from Welsh Drystan, influenced by French triste "sad" (see trist). The German form is Tristan.
trite (adj.)
"used till so common as to have lost its novelty and interest," 1540s, from Latin tritus "worn, oft-trodden," of language "much-used, familiar, commonplace," past participle adjective from terere "to rub, wear down" (from PIE root *tere- (1) "to rub, turn"). Related: Tritely; triteness.
triticale (n.)
hybrid cereal grass, 1952, from Modern Latin Triti(cum) "wheat" (literally "grain for threshing," from tritus, past participle of terere "to rub, thresh, grind," from PIE root *tere- (1) "to rub, turn") + (Se)cale "rye."
tritium (n.)
1933, Modern Latin, from Greek tritos "third" (see third) + chemical suffix -ium.
minor sea god, son of Poseidon and Amphitrite, from Latin Triton, from Greek Triton, cognate with Old Irish triath (genitive trethan) "sea."
triturate (v.)
"grind into powder," 1755, from Late Latin trituratus, past participle of triturare "to thresh, to grind," from Latin tritura "a rubbing, a threshing," from past participle stem of terere "to rub" (from PIE root *tere- (1) "to rub, turn"). Related: Triturated; triturating.
trituration (n.)
1640s, from Late Latin triturationem (nominative trituratio), noun of action from past participle stem of Latin triturare "to grind" (see triturate).
triumph (v.)
mid-15c., from Old French triumpher (13c.), from Latin triumphare, from triumphus (see triumph (n.)). Related: Triumphed; triumphing.
triumph (n.)
late 14c., "success in battle, conquest," also "spiritual victory" and "a procession celebrating victory in war," from Old French triumphe (12c., Modern French triomphe), from Latin triumphus "an achievement, a success; celebratory procession for a victorious general or admiral," from Old Latin triumpus, probably via Etruscan from Greek thriambos "hymn to Dionysus," a loan-word from a pre-Hellenic language.
triumphal (adj.)
early 15c., from Latin triumphalis, from triumphus (see triumph (n.)). Related: Triumphally.
triumphant (adj.)
early 15c., from Latin triumphantem (nominative triumphans), present participle of triumphare (see triumph (n.)). Related: Triumphantly.