theogony (n.) Look up theogony at Dictionary.com
1610s, "the account of the birth or genealogy of the gods," from Greek theogonia "generation or genealogy of the gods," from theos "a god" (see theo-) + -gonia "a begetting," from gonos "birth" (see genus).
theologian (n.) Look up theologian at Dictionary.com
late 15c., from Old French theologien (14c.), from theologie; see theology. A petty or paltry theologist is a theologaster (1620s), used in Medieval Latin by Martin Luther (1518).
theological (adj.) Look up theological at Dictionary.com
early 15c., "pertaining to theology," from Medieval Latin theologicalis, from Latin theologicus, from theologia (see theology). Related: Theologically.
theologist (n.) Look up theologist at Dictionary.com
1630s, from Medieval Latin theologista, agent noun from theologizare, from Latin theologia (see theology). Earlier in the same sense was theologician (1550s).
theology (n.) Look up theology at Dictionary.com
mid-14c., "the science of religion, study of God and his relationship to humanity," from Old French theologie "philosophical study of Christian doctrine; Scripture" (14c.), from Latin theologia, from Greek theologia "an account of the gods," from theologos "one discoursing on the gods," from theos "god" (see theo-) + -logos "treating of" (see -logy). Meaning "a particular system of theology" is from 1660s.
Theology moves back and forth between two poles, the eternal truth of its foundations and the temporal situation in which the eternal truth must be received. [Paul Tillich, "Systematic Theology," 1951]
theophany (n.) Look up theophany at Dictionary.com
"an appearance of God to man," 1630s, from Late Latin theophania, from Greek theos "god" (see theo-) + phainein "to show" (see phantasm). In Middle English "Epiphany" (late 12c.). Ancient Greek Theophaneia was the name of a festival at Delphi during which the statues of Apollo and other gods were displayed to the public.
Theophilus Look up Theophilus at Dictionary.com
masc. proper name, Latinized form of Greek Theophilos, literally "dear to God; loved by the gods," from theos "god" (see theo-) + philos "loved, beloved" (see -phile).
theorem (n.) Look up theorem at Dictionary.com
1550s, from Middle French théorème (16c.) and directly from Late Latin theorema, from Greek theorema "spectacle, sight," in Euclid "proposition to be proved," literally "that which is looked at," from theorein "to look at, behold" (see theory).
theoretical (adj.) Look up theoretical at Dictionary.com
1610s, "contemplative," with -al (1) + Late Latin theoreticus "of or pertaining to theory," from Greek theoretikos "contemplative, speculative, pertaining to theory" (by Aristotle contrasted to praktikos), from theoretos "that may be seen or considered," from theorein "to consider, look at" (see theory). Meaning "pertaining to theory, making deductions from theory not from fact" (opposed to practical) is from 1650s; earlier in this sense was theorical (c.1500). Meaning "ideal, hypothetical" is from 1790s (implied in theoretically). Related: theoretician.
theorist (n.) Look up theorist at Dictionary.com
"one given to theory and speculation," 1590s; see theory + -ist.
theorize (v.) Look up theorize at Dictionary.com
1630s, perhaps a formation in English from theory + -ize. Related: Theorized; theorizing.
theory (n.) Look up theory at Dictionary.com
1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theoria "contemplation, speculation; a looking at, viewing; a sight, show, spectacle, things looked at," from theorein "to consider, speculate, look at," from theoros "spectator," from thea "a view" (see theater) + horan "to see," possibly from PIE root *wer- (4) "to perceive" (see ward (n.)).

Earlier in this sense was theorical (n.), late 15c. Sense of "principles or methods of a science or art" (rather than its practice) is first recorded 1610s (as in music theory, which is the science of musical composition, apart from practice or performance). Sense of "an intelligible explanation based on observation and reasoning" is from 1630s.
theosophy (n.) Look up theosophy at Dictionary.com
1640s (implied in theosophical), "knowledge of divine things obtained through mystic study," from Medieval Latin theosophia (c.880), from Late Greek theosophia (c.500) "wisdom concerning God or things divine," from Greek theosophos "one wise about God," from theos "god" (see theo-) + sophia "skill, knowledge of, acquaintance with; philosophy," from sophos "wise, learned" (see sophist).

Applied variously over the years, including to the followers of Swedenborg. Taken as the name of a modern philosophical system (sometimes called Esoteric Buddhism), founded in New York 1875 as "Theosophical Society" by Madame Blavatsky and others, which has elements of Hinduism and Buddhism and claims supernatural knowledge of the divinity and his words deeper than that obtained from empiricism. Related: Theosophist.
ther- Look up ther- at Dictionary.com
often thero-, word-forming element meaning "beast," from comb. form of Greek ther "wild beast, beast of prey," from PIE *ghwer- "wild beast" (cognates: Latin ferus "wild," ferox "fierce;" see fierce). Also therio-, from Greek therion.
therapeutic (adj.) Look up therapeutic at Dictionary.com
pertaining to the healing of disease, 1640s, from Modern Latin therapeuticus "curing, healing," from Greek therapeutikos, from therapeutein "to cure, treat medically," primarily "do service, take care of, provide for," of unknown origin, related to therapon "attendant." Therapeutic was used from 1540s as a noun meaning "the branch of medicine concerned with treatment of disease." Related: Therapeutical (c.1600).
therapist (n.) Look up therapist at Dictionary.com
1880, from therapy + -ist; earlier was therapeutist (1816). Especially of psychotherapy practitioners from c.1930s.
therapy (n.) Look up therapy at Dictionary.com
1846, "medical treatment of disease," from Modern Latin therapia, from Greek therapeia "curing, healing, service done to the sick; a waiting on, service," from therapeuein "to cure, treat medically," literally "attend, do service, take care of" (see therapeutic).
there (adv., conj.) Look up there at Dictionary.com
Old English þær "in or at that place, so far as, provided that, in that respect," from Proto-Germanic *thær (cognates: Old Saxon thar, Old Frisian ther, Middle Low German dar, Middle Dutch daer, Dutch daar, Old High German dar, German da, Gothic þar, Old Norse þar), from PIE *tar- "there" (cognates: Sanskrit tar-hi "then"), from root *to- (see the) + adverbial suffix -r.

Interjectional use is recorded from 1530s, used variously to emphasize certainty, encouragement, or consolation. To have been there "had previous experience of some activity" is recorded from 1877.
thereabouts (adv.) Look up thereabouts at Dictionary.com
early 15c., "in that area, around there; mid-15c., "near to that time, approximately thence," from Old English þær onbutan "about that place" + adverbial genitive -es; see there + about.
thereafter (adv.) Look up thereafter at Dictionary.com
Old English þær æfter; see there + after. Similar formation in Dutch daarachter, Swedish derefter.
thereby (adv.) Look up thereby at Dictionary.com
Old English þærbig "thus, by means of or because of that;" see there + by. Similar formation in Old Frisian therbi, Middle Low German darbi, German dabei, Dutch daarbij.
therefor (adv.) Look up therefor at Dictionary.com
"for this, for that," Middle English variant spelling of therefore (q.v.); in modern use perhaps perceived as there + for.
therefore (adv.) Look up therefore at Dictionary.com
Old English þærfore; from there + fore, Old English and Middle English collateral form of for. Since c.1800, therefor has been used in sense of "for that, by reason of that;" and therefore in sense of "in consequence of that." Similar formation in Dutch daarfoor, German dafür, Danish derfor.
therefrom (adv.) Look up therefrom at Dictionary.com
mid-13c., there from. One word from 17c.; see there + from.
therein (adv.) Look up therein at Dictionary.com
"in that place, time, or thing," Old English þærin; see there + in. Similar formation in German darin.
theremin (n.) Look up theremin at Dictionary.com
1927, from the name of its inventor, Russian engineer Léon Thérémin (1896-1993).
thereof (adv.) Look up thereof at Dictionary.com
"of that, of it," Old English þærof; see there + of. Similar formation in Swedish, Danish deraf.
thereon (adv.) Look up thereon at Dictionary.com
Old English þæron; see there + on. Similar formation in German daran.
Theresa Look up Theresa at Dictionary.com
also Teresa, fem. proper name, from French Thérèse, from Latin Therasia, apparently from Greek Therasia, name of two volcanic islands, one near Sicily, one near Crete. In the top 50 most popular names for girls born in the U.S. from 1953 to 1969.
thereto (adv.) Look up thereto at Dictionary.com
Old English þærto "to it, in that place, for that purpose, belonging to;" see there + to. Similar formation in Old Saxon tharto, Old High German darazuo, German dazu.
thereunder (adv.) Look up thereunder at Dictionary.com
Old English þærunder; see there + under. Similar formation in Old Frisian therunder, German darunter.
thereupon (adv.) Look up thereupon at Dictionary.com
late 12c., þer uppon; see there + upon.
therewith (adv.) Look up therewith at Dictionary.com
c.1200, "along with, in company with," from there + with. Old English þær wiþ meant "against, in exchange for." Similar formation in Swedish dervid, Danish derved.
thermal (adj.) Look up thermal at Dictionary.com
1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat, feverish heat," from PIE *gwher- "to heat, warm" (cognates: Latin fornax "an oven, kiln," formus "warm," Old English wearm; see warm (adj.)). Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.
thermo- Look up thermo- at Dictionary.com
before vowels therm-, word-forming element meaning "hot, heat, temperature," used in scientific and technical words, from comb. form of Greek thermos "hot, warm," therme "heat" (see thermal).
thermochemistry (n.) Look up thermochemistry at Dictionary.com
also thermo-chemistry, 1840, from thermo- + chemistry.
thermocline (n.) Look up thermocline at Dictionary.com
1897, from thermo- + -cline, from Greek klinein "to slope" (see lean (v.)).
thermocouple (n.) Look up thermocouple at Dictionary.com
also thermo-couple, 1862, from thermo-electric + couple (n.).
thermodynamic (adj.) Look up thermodynamic at Dictionary.com
1849, from thermo- + dynamic (adj.).
thermodynamics (n.) Look up thermodynamics at Dictionary.com
theory of relationship between heat and mechanical energy, 1854, from thermodynamic (adj.); also see -ics. "The consideration of moving forces, though suggested by the form of the word, does not enter into the subject to any considerable extent" [Century Dictionary].
thermograph (n.) Look up thermograph at Dictionary.com
"automatic self-registering thermometer," 1881, from thermo- + -graph "instrument for recording; something written." Related: Thermographic.
thermography (n.) Look up thermography at Dictionary.com
1840, "method of writing which requires heat to develop the characters," from thermo- + -graphy.
thermometer (n.) Look up thermometer at Dictionary.com
1630s, from French thermomètre (1620s), coined by Jesuit Father Jean Leuréchon from Greek thermos "hot" (see thermal) + metron "measure" (see meter (n.2)). An earlier, Latinate form was thermoscopium (1610s). The earliest such device was Galileo's air-thermometer, invented c.1597. The typical modern version, with mercury in glass, was invented by Fahrenheit in 1714. Related: Thermometric; thermometrical.
thermonuclear (adj.) Look up thermonuclear at Dictionary.com
1938 with reference to stars, 1953 of weapons (technically only to describe the hydrogen bomb), from thermo- + nuclear.
thermoplastic (adj.) Look up thermoplastic at Dictionary.com
1870, see thermo- + plastic (adj.). As a noun from 1929.
Thermopylae Look up Thermopylae at Dictionary.com
narrow land passage along the Malian Gulf in ancient Greece, from Greek thermos "hot" (see thermal) + pylai, plural of pyle "gate; mountain pass, entrance into a region" (see pylon). In reference to nearby hot sulfur springs. Often simply hai pylai "the gates." Figurative of heroic resistance against overwhelming numbers since the battle fought there between the Greeks and Persians in 480 B.C.E.
Thermos (n.) Look up Thermos at Dictionary.com
trademark registered in Britain 1907, invented by Sir James Dewar (patented 1904 but not named then), from Greek thermos "hot" (see thermal). Dewar built the first one in 1892, but it was first manufactured commercially in Germany in 1904, when two glass blowers formed Thermos GmbH. Supposedly the company sponsored a contest to name the thing, and a Munich resident won with a submission of Thermos.
thermosphere (n.) Look up thermosphere at Dictionary.com
1924, from thermo- + sphere.
thermostat (n.) Look up thermostat at Dictionary.com
automatic instrument for regulating temperature, 1831, from thermo- + -stat.
Theropoda (n.) Look up Theropoda at Dictionary.com
order of dinosaurs, from ther- "beast" + podos genitive of pous "foot" (see foot (n.)). So called because the structure of the feet resembled quadrupeds rather than birds. Related: Theropod.