introspection (n.)
1670s, noun of action from past participle stem of Latin introspicere "to look into, look at," from intro- "inward" (see intro-) + specere "to look at" (see scope (n.1)).
introspective (adj.)
1820, from Latin introspect-, past participle stem of introspicere (see introspection) + -ive.
introversion (n.)
1650s, of thought or contemplation, from Modern Latin introversionem, noun of action from past participle stem of *introvertere (see introvert). Meaning "tendency to withdraw from the world" is from 1912.
introvert (v.)
1650s, from Latin intro- "inward" (see intro-) + vertere "to turn" (see versus). The noun, "introverted person" (opposed to extrovert) is 1918, from German psychology, introduced there by C.G. Jung (1875-1961).
introverted (adj.)
1781, "directed inward," past participle adjective from introvert. Psychological sense is from 1915.
intrude (v.)
early 15c., back-formation from intrusion, or else from Latin intrudere "to thrust in" (see intrusion). Related: Intruded; intruding.
intruder (n.)
1530s, agent noun from intrude. Originally legal.
intrusion (n.)
late 14c., from Old French intrusion (14c.), from Medieval Latin intrusionem (nominative intrusio) "a thrusting in," noun of action from past participle stem of Latin intrudere, from in- "in" (see in- (2)) + trudere "to thrust, push" (see extrusion).
intrusive (adj.)
c.1400, from Latin intrus-, past participle stem of intrudere (see intrusion) + -ive. Related: Intrusively; intrusiveness.
intubate (v.)
1610s, "to form into tubes," from in- (2) "in" + Latin tuba "tube" (see tuba) + -ate (2). Medical sense is from 1889. Related: Intubated; intubation.
intuit (v.)
1776, "to tutor," from Latin intuit-, past participle stem of intueri (see intuition). Meaning "to perceive directly without reasoning" is from 1840, in this sense perhaps a back-formation from intuition. Related: Intuited; intuiting.
intuition (n.)
mid-15c., from Late Latin intuitionem (nominative intuitio) "a looking at, consideration," noun of action from past participle stem of Latin intueri "look at, consider," from in- "at, on" (see in- (2)) + tueri "to look at, watch over" (see tuition).
intuitive (adj.)
1640s, from Middle French intuitif or directly from Medieval Latin intuitivus, from intuit-, past participle stem of intueri "look at, consider" (see intuition). Related: Intuitively; intuitiveness.
intumescence (n.)
1650s, from French intumescence, from Latin intumescere (see intumescent).
intumescent (adj.)
1796, from Latin intumescentem (nominative intumescens), present participle of intumescere "to swell up," from in- "in" (see in- (2)) + tumescere (see tumescence).
inturn (n.)
1590s, "turning in of the toes" (especially in dancing), from in + turn.
intussusception (n.)
1707, literally "a taking in," from Latin intus "within" (see ento-) + susceptionem "a taking up" (see susceptible).
inundate (v.)
1620s, back-formation from inundation, or else from Latin inundatus, past participle of inundare "to overflow, run over" (see inundation). Related: Inundated; inundating.
inundation (n.)
early 15c., from Latin inundationem (nominative inundatio) "an overflowing," noun of action from past participle stem of inundare "to overflow," from in- "onto" (see in- (2)) + undare "to flow," from unda "wave" (see water (n.1)).
inure (v.)
early 15c., in ure "in practice," from obsolete ure "work, practice, exercise, use," probably from Old French uevre, oeuvre "work," from Latin opera (see opus). Related: Inured; inuring.
inutile (adj.)
late 15c., from French inutile (12c., inutele), from Latin inutilis "useless, unprofitable," from in- "not" (see in- (1)) + utilis (see utility).
inutility (n.)
1590s, from Middle French inutilité (15c.), from Latin inutilitatem (nominative inutilitas) "uselessness," from inutilis (see inutile).
invade (v.)
late 15c., from Middle French invader "to invade," and directly from Latin invadere "to go into, enter upon; assail, assault, attack" (see invasion). Related: invaded; invading.
invader (n.)
1540s, agent noun from invade.
invaginate (v.)
1650s, from Medieval Latin invaginatus, past participle of invaginare "to put into a sheath," from Latin in- "in" (see in- (2)) + vagina "a sheath" (see vagina). Related: Invaginated; invagination.
invalid (adj.1)
"not strong, infirm," 1640s, from Latin invalidus "not strong, infirm, weak, feeble," from in- "not" (see in- (1)) + validus "strong" (see valid). Specific meaning "infirm from sickness, disease, or injury" is from 1640s.
invalid (adj.2)
"of no legal force," 1630s, from special use of Latin invalidus (see invalid (adj.1)).
invalid (n.)
1709, originally of disabled military men, from invalid (adj.1). Invalides is short for French Hôtel des Invalides, home for old and disabled soldiers in the 7th arrondissement of Paris.
invalidate (v.)
1640s, from invalid + -ate (2). Related: Invalidated; invalidating.
invalidation (n.)
1771, noun of action from invalidate (v.).
invalidity (n.)
1540s, from Latin invalidatus (see invalid (adj.)).
invaluable (adj.)
1570s, "above value," from in- (1) "not" + value (v.) "estimate the worth of" + -able. It also has been used in a sense "without value, worthless" (1630s).
invariability (n.)
1640s, from invariable + -ity.
invariable (adj.)
early 15c., from Old French invariable (14c.) and directly from Medieval Latin invariabilis, from in- "not, opposite of" (see in- (1)) + variabilis (see variable). Related: Invariably.
invariant (adj.)
1851, from in- (1) "not" + variant.
invasion (n.)
mid-15c., from Old French invasion "invasion, attack, assaut" (12c.), from Late Latin invasionem (nominative invasio) "an attack, invasion," noun of action from past participle stem of Latin invadere "go into, fall upon, attack, invade," from in- "in" (see in- (2)) + vadere "go, walk" (see vamoose).
invasive (adj.)
mid-15c., from Middle French invasif (15c.), from Medieval Latin invasivus, from invas-, past participle stem of invadere (see invasion).
invective (n.)
1520s, from Medieval Latin invectiva "abusive speech," from Late Latin invectivus "abusive," from Latin invectus, past participle of invehi "to attack with words" (see inveigh). For nuances of usage, see humor. The earlier noun form was inveccion (mid-15c.).
inveigh (v.)
late 15c., "to introduce," from Latin invehi "to attack with words," originally "carry oneself against," passive infinitive of invehere "bring in, carry in," from in- "against" (see in- (1)) + vehere "to carry" (see vehicle). Meaning "to give vent to violent denunciation" is from 1520s. Related: Inveighed; inveighing.
inveigle (v.)
late 15c., "to blind (someone's) judgment," alteration of Middle French aveugler "delude, make blind," from Vulgar Latin *aboculus "without sight, blind," from Latin ab- "without" (see ab-) + oculus "eye" (see eye (n.)). Loan-translation of Greek ap ommaton "without eyes." Meaning "to win over by deceit, seduce" is 1530s.
invent (v.)
late 15c., "find, discover," a back-formation from invention or else from Latin inventus, past participle of invenire “to come upon; devise, discover” (see invention). Meaning "make up, think up" is from 1530s, as is that of "produce by original thought." Related: Invented; inventing.
invention (n.)
c.1400, "devised method of organization," from Old French invencion (13c.) and directly from Latin inventionem (nominative inventio) "faculty of invention; a finding, discovery," noun of action from past participle stem of invenire "devise, discover, find," from in- "in, on" (see in- (2)) + venire "to come" (see venue).

Meaning "finding or discovering of something" is early 15c. in English; sense of "thing invented" is first recorded 1510s. Etymological sense preserved in Invention of the Cross, Church festival (May 3) celebrating the reputed finding of the Cross of the Crucifixion by Helena, mother of Constantine, in 326 C.E.
inventive (adj.)
early 15c., "skilled in invention," from Old French inventif (15c.), from Latin invent-, past participle stem of invenire (see invention). Related: Inventively; inventiveness.
inventor (n.)
c.1500, "a discoverer," from Latin inventor (fem. inventrix) "contriver, author, discoverer," agent noun from past participle stem of invenire (see invention). Meaning "one who contrives or produces a new thing or process" is from 1550s.
inventory (n.)
early 15c., from Old French inventoire "inventory, detailed list of goods, catalogue," from Medieval Latin inventorium (Late Latin inventarium) "list of what is found," from Latin inventus, past participle of invenire "to find" (see invention). The verb is first recorded c.1600, from the noun.
inverse (adj.)
mid-15c., from Latin inversus, past participle of invertere (see invert). Related: Inversely. As a noun, 1680s, from the adjective.
inversion (n.)
1550s, from Latin inversionem (nominative inversio) "an inversion," noun of action from past participle stem of invertere (see invert).
invert (v.)
1530s, from Middle French invertir or directly from Latin invertere "turn upside down, turn about," from in- "in, on" (see in- (2)) + vertere "to turn" (see versus). Related: Inverted; inverting; invertedly.
invertebrate (n.)
1826, from Latin in- "not" (see in- (1)) + vertebra "joint" (see vertebra). Invertebrata as a biological classification was coined 1805 by French naturalist Georges Léopole Chrétien Frédéric Dagobert, Baron Cuvier (1769-1832). As an adjective by 1838.
invest (v.)
late 14c., "to clothe in the official robes of an office," from Latin investire "to clothe in, cover, surround," from in "in, into" (see in- (2)) + vestire "to dress, clothe" (see wear (v.)). The meaning "use money to produce profit" first attested 1610s in connection with the East Indies trade, and is probably a borrowing of Italian investire (13c.) from the same Latin root, via the notion of giving one's capital a new form. The military meaning "to besiege" is from c.1600. Related: Invested; investing.