epochal (adj.)
1680s, from epoch + -al (1).
epode (n.)
1590s, a kind of lyric poem in which a short line follows a longer one (invented by Archilochus, also used by Horace), from Latin epodos, from Greek epodus "after-song, incantation," from epi "after" (see epi-) + odein "to sing" (see ode).
eponym (n.)
one whose name becomes that of a place, a people, an era, an institution, etc., 1846, from Greek eponymos "given as a name, giving one's name to something," from epi "upon" (see epi-) + onyma, Aeolic dialectal variant of onoma "name" (see name (n.)).
eponymous (adj.)
1846; see eponym + -ous.
epoxy (n.)
1916, in reference to certain chemical compounds, from epi- + first element of oxygen (epoxy- is used as a prefix in chemistry). Resins from them are used as powerful glues. Hence the verb meaning "to bond with epoxy" (1965). Related: Epoxied.
epsilon (n.)
from Greek, literally e psilon "bare -e-, -e- and nothing else," in contradistinction to the diphthong -ai-, which has the same sound. Greek psilon "smooth, simple" is of uncertain origin.
Epsom salts
magnesium sulphate, 1770, obtained from Epsom water, the water of a mineral spring at Epsom in Surrey, England, the medicinal properties of which were discovered in Elizabethan times. The place name is recorded c.973 as Ebbesham, literally "Ebbi's homestead," from the name of some forgotten Anglo-Saxon. The mineral supply there was exhausted 19c.
Epstein-Barr virus
1968, named for British virologist Michael Anthony Epstein and Irish-born virologist Yvonne M. Barr.
ept (adj.)
1938, back-formation from inept, usually with an attempt at comical effect. Related: Eptitude; eptly.
equability (n.)
1530s, from Latin aequabilitas, from aequabilis (see equable).
equable (adj.)
1670s, back-formation from equability or else from Latin aequabilis "equal, consistent, uniform," from aequare "make uniform" (see equate).
equal (adj.)
late 14c., from Latin aequalis "uniform, identical, equal," from aequus "level, even, just," of unknown origin. Parallel formation egal (from Old French egal) was in use late 14c.-17c. The noun is recorded from 1570s.
equal (v.)
1580s, "compare, liken," also "match, rival," from equal (adj.). Related: Equaled; equaling.
equalitarian (adj.)
1799, in reference to the doctrine that all mankind are equal, from equality on model of humanitarian, etc.
equality (n.)
late 14c., "evenness of surface, uniformity of size;" c.1400, in reference to amount or number, from Old French equalité (Modern French égalité, which form dates from 17c.), from Latin aequalitatem (nominative aequalitas) "equality, similarity, likeness" (also sometimes with reference to civil rights), from aequalis (see equal). Of privileges, rights, etc., from 1520s.
equalization (n.)
1793, from equalize + -ation.
equalize (v.)
1580s, from equal + -ize. Related: Equalized; equalizing.
equalizer (n.)
1792, agent noun from equalize. Sports sense attested by 1930; in the U.S. underworld slang sense of "pistol," it is from c.1900.
equally (adv.)
late 14c., from equal + -ly (2).
equanimity (n.)
c.1600, "fairness, impartiality," from French équanimité, from Latin aequanimitatem (nominative aequanimitas) "evenness of mind, calmness," from aequus "even, level" (see equal (adj.)) + animus "mind, spirit" (see animus). Meaning "evenness of temper" in English is from 1610s.
equanimous (adj.)
1650s, from Latin aequanimis (see equanimity) + -ous.
equate (v.)
early 15c., from Latin aequatus "level, levelled, even," past participle of aequare "make even or uniform, make equal," from aequus "level, even, equal" (see equal (adj.)). Earliest use in English was of astrological calculation, then "to make equal;" meaning "to regard as equal" is early 19c. Related: Equated; equating.
equation (n.)
late 14c., a term in astrology; meaning "action of making equal" is from 1650s; mathematical sense is from 1560s, on notion of equalizing the expressions; from Latin aequationem (nominative aequatio) "an equal distribution, community," from past participle stem of aequare (see equal (adj.)). Chemistry sense is from 1807.
equator (n.)
late 14c., from Medieval Latin aequator diei et noctis "equalizer of day and night" (when the sun is on the celestial equator, twice annually, day and night are of equal length), agent noun from Latin aequare "make equal" (see equate). Sense of "celestial equator" is earliest, extension to "terrestrial line midway between the poles" first recorded in English 1610s.
equatorial (adj.)
1660s, from equator + -al (1).
equerry (n.)
1590s, short for groom of the equirrie, from esquiry "stables" (1550s), from Middle French escuerie (Modern French écurie), perhaps from Medieval Latin scuria "stable," from Old High German scura "barn;" or from Old French escuier "groom," from Vulgar Latin scutarius "shield-bearer." In either case, spelling influenced by Latin equus "horse," which is unrelated.
equestrian (adj.)
1650s, formed in English from Latin equester (genitive equestris) "of a horseman," from eques "horseman, knight," from equus "horse" (see equine). As a noun, "one who rides on horseback," from 1791. The pseudo-French fem. equestrienne is attested from 1848.
equi-
word-forming element meaning "equal," from Latin aequi-, comb. form of aequus "equal, even" (see equal (adj.)).
equiangular (adj.)
1650s; see equi- + angular.
equidistant (adj.)
1560s, from French équidistant (14c.), from Late Latin aequidistantem (nominative aequidistans), from aequi- (see equal) + distantem (see distant). In reference to a type of map projection, from 1866.
equilateral (adj.)
1560s, from Late Latin aequilateralis, from aequi- (see equal (adj.)) + lateralis (see lateral).
equilibrium (n.)
c.1600, from Latin aequilibrium, from aequus "equal" (see equal) + libra "a balance, scale, plummet" (see Libra).
equine (adj.)
1765, from Latin equinus, from equus "horse," from PIE root *ekwo- "horse" (cognates: Greek hippos, Old Irish ech, Old English eoh, Gothic aihwa-, Sanskrit açva-, Avestan aspa-, Old Church Slavonic ehu-, all meaning "horse").
equinox (n.)
late 14c., from Old French equinoce (12c.) or directly from Medieval Latin equinoxium "equality of night (and day)," from Latin aequinoctium "the equinoxes," from aequus "equal" (see equal (adj.)) + nox (genitive noctis) "night" (see night). The Old English translation was efnniht. Related: Equinoctial.
equip (v.)
1520s, from Middle French équiper "to fit out," from Old French esquiper "fit out a ship" (12c.), probably from Old Norse skipa "fit out a ship," from skip "ship" (see ship (n.)). Related: Equipped; equipping. Spanish and Portuguese esquipar are from French.
equipage (n.)
1570s, from French équipage (15c.), from équiper (see equip). Now largely replaced by equipment.
equiparation (n.)
from Latin aequiparationem (nominative aequiparatio) "an equalizing, comparison," from aequiparare "put on equality, compare," from aequipar "equal, alike," from aequus "equal, even" (see equal (adj.)) + par (see par (n.)). Related: Equiparate.
equipment (n.)
1717, "things equipped;" 1748, "action of equipping;" from equip + -ment, or from French équipement. Superseding earlier equipage.
equipoise (n.)
1650s, a contraction of the phrase equal poise (1550s); see equal (adj.) + poise (n.).
equitable (adj.)
1640s, from French équitable (16c.), from équité (see equity). Related: Equitably.
equity (n.)
early 14c., from Old French equite (13c.), from Latin aequitatem (nominative aequitas) "equality, conformity, symmetry, fairness," from aequus "even, just, equal" (see equal). As the name of a system of law, 1590s, from Roman naturalis aequitas, the general principles of justice which corrected or supplemented the legal codes.
equivalence (n.)
1540s, from French équivalence, from Medieval Latin aequivalentia, from aequivalentem (see equivalent). Related: Equivalency (1530s).
equivalent (adj.)
early 15c., from Middle French equivalent and directly from Late Latin aequivalentem (nominative aequivalens) "equivalent," present participle of aequivalere "be equivalent," from Latin aequus "equal" (see equal) + valere "be well, be worth" (see valiant). As a noun from c.1500.
equivocal (adj.)
c.1600, from Late Latin aequivocus "of equal voice, of equal significance, ambiguous" (see equivocation) + -al (1). Earlier in same sense was equivoque (late 14c.). Related: Equivocally (1570s).
equivocate (v.)
early 15c., equivocaten, from Medieval Latin equivocatus, past participle of equivocare "to call by the same name," from Late Latin aequivocus (see equivocation). Related: Equivocated; equivocating.
equivocation (n.)
late 14c., "the fallacy of using a word in different senses at different stages of the reasoning" (a loan-translation of Greek homonymia, literally "having the same name"), from Old French equivocation, from Late Latin aequivocationem (nominative aequivocatio), from aequivocus "of identical sound," past participle of aequivocare, from aequus "equal" (see equal (adj.)) + vocare "to call" (see voice (n.)).
equivocator (n.)
1590s, from Late Latin aequivocator, agent noun from aequivocare (see equivocation).
er
as a sound of hesitation or uncertainty, attested from mid-19c.
ER
abbreviation of emergency room, by 1965.
era (n.)
1716, earlier aera (1610s), from Late Latin aera, era "an era or epoch from which time is reckoned," probably identical with Latin aera "counters used for calculation," plural of aes (genitive aeris) "brass, copper, money" (see ore, also compare copper).

The Latin word's use in chronology said to have begun in 5c. Spain (where, for some reason unknown to historians, the local era, aera Hispanica, began 38 B.C.E.; some say it was because of a tax levied that year). Like epoch, in English it originally meant "the starting point of an age;" meaning "system of chronological notation" is c.1640s; that of "historical period" is from 1741, as in the U.S. Era of Good Feeling (which was anything but) in reference to the Monroe Administration (1817-24), attested from 1817.