Online Etymology Dictionary

theology (n.): mid-14c., from Old French theologie "philosophical treatment of Christian doctrine" (14c.), from Latin theologia, from Greek theologia "an account of the gods," from theologos "one discoursing on the gods," from theos "god" (see Thea) + -logos "treating of."
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.): 1630s, from Late Latin theophania, from Greek theophaneia, from theos "god" (see Thea) + phainein "to show" (see phantasm).
Theophilus: masc. proper name, Greek Theophilos, literally "dear to God; loved by the gods," from theos "god" (see Thea) + philos "loved, beloved" (see -phile).
theorem (n.): 1550s, from Middle French théorème, from Late Latin theorema, from Greek theorema "spectacle, speculation," in Euclid "proposition to be proved," from theorein "to consider" (see theory).
theoretical (adj.): 1610s, from Late Latin theoreticus "of or pertaining to theory," from Greek theoretikos "contemplative, pertaining to theory" (by Aristotle contrasted to praktikos), from theoretos "that may be seen or considered," from theorein "to consider, look at" (see theory). Related: Theoretically.
theorist (n.): 1590s; see theory + -ist.
theorize (v.): 1630s, perhaps a formation in English from theory + -ize. Related: Theorized; theorizing.
theory (n.): 1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theoria "contemplation, speculation, a looking at, things looked at," from theorein "to consider, speculate, look at," from theoros "spectator," from thea "a view" + horan "to see" (see warrant). Sense of "principles or methods of a science or art (rather than its practice)" is first recorded 1610s. That of "an explanation based on observation and reasoning" is from 1630s.
theosophy (n.): 1640s (implied in theosophical), "knowledge about God and nature obtained through mystical study," from Medieval Latin theosophia (c.880), from Late Greek theosophia (c.500, Pseudo-Dionysus) "wisdom concerning God or things divine," from Greek theosophos "one wise about God," from theos "god" (see Thea) + sophos "wise, learned" (see sophist). 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 combines teachings of Hinduism and Buddhism.
therapeutic (adj.): pertaining to the healing of disease, 1640s, probably shortened from therapeutical (c.1600), from Modern Latin therapeuticus "curing, healing," from Greek therapeutikos, from therapeutes "one ministering," from therapeutein "to cure, treat," of unknown origin, related to therapon (genitive therapontos) "attendant." Therapeutic was used from 1540s as a noun meaning "the branch of medicine concerned with treatment of disease."
therapist (n.): 1886, from therapy + -ist; earlier was therapeutist (1816). Especially of psychotherapy practitioners from c.1930s.
therapy (n.): 1846, "medical treatment of disease," from Modern Latin therapia, from Greek therapeia "curing, healing," from therapeuein "to cure, treat medically," literally "attend, do service, take care of;" related to therapon "servant, attendant."
there (adv.): Old English þær "in or at that place," from Proto-Germanic *thær (cf. 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" (cf. Sanskrit tar-hi "then"), from root *to- (see the) + adverbial suffix -r.

Interjectional use is recorded from 1530s. To have been there "had previous experience of some activity" is recorded from 1877.
thereabouts (adv.): c.1400, from Old English þærabutan + adverbial genitive -es; see there + about.
thereafter (adv.): Old English þær æfter; see there + after. Cf. Dutch daarachter, Swedish derefter.
thereby (adv.): Old English þærbig "by means of or because of that;" see there + by. Cf. German dabei, Dutch daarbij.
therefor (adv.): see therefore.
therefore (adv.): 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."
therefrom (adv.): mid-13c., there from. One word from 17c.; see there + from.
therein (adv.): Old English þærin; see there + in.
theremin (n.): 1927, from the name of its inventor, Russian engineer Léon Thérémin (1896-1993).
thereof (adv.): Old English þær of; see there + of.
thereon (adv.): Old English þæron; see there + on.
Theresa: fem. proper name, from French Thérèse, from Latin Therasia, apparently from Greek Therasia, name of two volcanic islands, one near Sicily, one near Crete.
thereto (adv.): Old English þærto; see there + to.
thereunder (adv.): Old English þærunder; see there + under.
thereupon (adv.): late 12c., þer uppon; see there + upon.
therewith: Old English þær wiþ; see there + with.
thermal (adj.): 1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat," from PIE *ghwerm-/*ghworm- "warm" (cf. Latin fornax "an oven, kiln," formus "warm," Old English wearm; see warm). 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-: word-forming element meaning "hot, heat," used in scientific and technical words, from comb. form of Greek thermos "hot," therme "heat" (see thermal).
thermochemistry (n.): 1844, from thermo- + chemistry.
thermocline (n.): 1898, from thermo- + -cline, from Greek klinein "to slope" (see lean (v.)).
thermocouple (n.): 1890, from thermo- + couple.
thermodynamic (adj.): 1849, from thermo- + dynamic (adj.).
thermodynamics (n.): theory of relationship between heat and mechanical energy, 1854, from adj. thermodynamic; also see -ics.
thermograph (n.): 1840, from thermo- + -graph "instrument for recording; something written," from Greek graphe "writing," from graphein "to write, express by written characters," earlier "to draw, represent by lines drawn" (see -graphy). Related: Thermographic.
thermometer (n.): 1630s, from French thermomètre (1620s), coined by Jesuit Father Leuréchon from Greek thermos "hot" (see thermal) + metron "measure" (see -meter). An earlier, Latinate form was thermoscopium (1610s). The earliest such device was Galileo's air-thermometer, invented c.1597.
thermonuclear (adj.): 1938 with reference to stars, 1953 of weapons (technically only to describe the hydrogen bomb), from thermo- + nuclear.
thermoplastic (adj.): 1883, see thermo- + plastic. As a noun from 1929.
Thermopylae: 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."
Thermos (n.): 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.): 1924, from thermo- + sphere.
thermostat (n.): 1831, from thermo- + -stat.
thero-: word-forming element meaning "beast," from comb. form of Greek ther "wild beast," from PIE *ghwer- "wild beast" (cf. Latin ferus "wild," ferox "fierce;" see fierce).
thesaurus (n.): 1823, "treasury, storehouse," from Latin thesaurus "treasury, treasure," from Greek thesauros "a treasure, treasury, storehouse, chest," from root of tithenai "to put, to place." The meaning "encyclopedia filled with information" is from 1840, but existed earlier as thesaurarie (1590s), used as a title by early dictionary compilers. Meaning "collection of words arranged according to sense" is first attested 1852 in Roget's title. Thesaur is attested in Middle English with the meaning "treasure" (15c.-16c.).
these: Old English þæs, variant of þas, nominative and accusative plural of þes, þeos, þis "this" (see this).
Theseus: legendary hero-king of Athens.
thesis (n.): late 14c., "unaccented syllable or note," from Latin thesis "unaccented syllable in poetry," later "stressed part of a metrical foot," from Greek thesis "a proposition," also "downbeat" (in music), originally "a setting down or placing," from root of tithenai "to place, put, set," from PIE root *dhe- "to put, to do" (see factitious). Sense in logic of "a proposition, statement to be proved" is first recorded 1570s; that of "dissertation written by a candidate for a university degree" is from 1650s.
thespian (adj.): 1670s, "of or pertaining to tragedy or dramatic acting," from Greek Thespis, poet of 6c. B.C.E., the traditional father of Greek tragedy. The names is literally "inspired by the gods."
thespian (n.): "an actor," 1827, from thespian (adj.). Short form thesp is attested from 1962.