Online Etymology Dictionary

intern (n.): "one working under supervision as part of professional training," originally "assistant resident physician, doctor in training in a hospital," 1879, American English, from French interne "assistant doctor," noun use of interne "internal," from Latin internus "within, inward" (see intern (v.)). Extended to other professions (originally teaching) from 1963 in reference to one under training and acquiring practical experience.
intern (v.2): "to act or serve as as a (medical) intern," 1933, from intern (n.). Related: Interned; interning.
internal (adj.): early 15c., "extending toward the interior," from Medieval Latin internalis, from Latin internus "within, inward, internal," figuratively "domestic," expanded from pre-Latin *interos, *interus "on the inside, inward," from PIE *en-ter- (source also of Old Church Slavonic anter, Sanskrit antar "within, between," Old High German unter "between," and the "down" sense of Old English under); suffixed (comparative) form of *en "in" (see in (adv.)).

Meaning "situated within" is from 1590s. Meaning "of or pertaining to the domestic affairs of a country (as in internal revenue) is from 1795; the notion is "pertaining to the subject itself; independent of others." Internal-combustion in reference to an engine in which fuel is burned inside it, is from 1884. Related: Internally.
internality (n.): 1802, from internal + -ity.
internalization (n.): 1853, from internal + -ization.
internalize (v.): 1856, American English, from internal + -ize. Related: Internalized; internalizing.
international (adj.): 1780, coined by Jeremy Bentham from inter- "between" + national (adj.). In the phrase international jurisprudence. He footnotes the word with:
The word international, it must be acknowledged, is a new one; though, it is hoped, sufficiently analogous and intelligible. It is calculated to express, in a more significant way, the branch of law which goes commonly under the name of the law of nations: an appellation so uncharacteristic, that, were it not for the force of custom, it would seem rather to refer to internal jurisprudence. [Bentham, "Introduction to the Principles of Morals and Legislation"]
As a noun and with a capital -i-, it is short for International Working Men's Association, a socialistic worker's movement with global aims, the first chapter of which was founded in London by Marx in 1864. The group lends its name to "The Internationale" (from fem. of French international, which is from English), the socialist hymn, written 1871 by Eugène Pottier. International Dateline is from 1882. Related: Internationally (1821).
internationalise (v.): chiefly British English spelling of internalize (q.v.). For spelling, see -ize. Related: Internationalisation.
internationalism (n.): 1851, from international + -ism. Related: Internationalist.
internationalization (n.): 1860, with reference to law; see international + -ization.
internationalize (v.): 1864, from international (adj.) + -ize. Related: Internationalized; internationalizing.
internecine (adj.): 1660s, "deadly, destructive," from Latin internecinus "very deadly, murderous, destructive," from internecare "kill or destroy," from inter (see inter-) + necare "kill" (see noxious).

Considered by OED as misinterpreted in Johnson's Dictionary [1755], which defined it as "endeavouring mutual destruction," but a notion of "mutually destructive" has been imported into the word in English because in English inter- usually conveys the idea of "mutual." The Latin prefix is said to have had here only an intensive sense; "the Latin word meant merely of or to extermination ... without implying that of both parties" [Fowler].
internee (n.): "one who is interned," from intern (v.1) + -ee.
Internet (n.): 1984, "the linked computer networks of the U.S. Defense Department," shortened from internetwork, inter-network, which was used from 1972 in reference to (then-hypothetical) networks involving many separate computers. From inter- "between" + network (n.).
interneuron (n.): 1939, from neuron + first element from internuncial (adj.) "communicating between different parts of the body," from Latin internuncius "a messenger, mediator," from inter- "between" (see inter-) + nuntius "messenger," from PIE root *neu- "to shout" (source also of Greek neuo "to nod, beckon," Old Irish noid "make known").
internist (n.): 1897, from internal (medicine) + -ist.
internment (n.): 1840, "confinement within a place," from intern (v.1) + -ment. Compare French internement. Internment camp is attested from 1916.
internship (n.): 1899, from intern (n.) + -ship.
interoffice (adj.): also inter-office, 1864, from inter- "between" + office (n.).
interoperable (adj.): 1969, from inter- "between" + operable. Related: Interoperability.
interpellate (v.): 1590s, "interrupt," from Latin interpellatus, past participle of interpellare "to interrupt by speaking" (see interpellation). Parliamentary sense of "question formally or publically" is from 1874, from French. Related: Interpellated; interpellating.
interpellation (n.): late 15c., "an appeal" (obsolete), from Latin interpellationem (nominative interpellatio) "an interruption," noun of action from past participle stem of interpellare "to interrupt by speaking," from inter "between" (see inter-) + pellare, collateral form of pellere "to drive" (see pulse (n.1)). The parliamentary sense is attested in English from 1837, from French.
interpenetrate (v.): 1809, from inter- "between" + penetrate (v.). Related: Interpenetrated; interpenetrating.
interpenetration (n.): 1809, from inter- + penetration.
interpersonal (adj.): 1911 (OED finds an isolated use from 1842), from inter- "between" + person (n.) + -al. Introduced in psychology (1938) by H.S. Sullivan (1892-1949) to describe behavior between people in an encounter. Related: Interpersonally.
interphase (n.): in cytology, 1913, from German interphase (1912); see inter- "between" + phase (n.).
interplanetary (adj.): 1690s, "existing between planets," from inter- "between" + planet + -ary. In reference to travel between planets, attested from 1897. Boyle and Locke both used intermundane in the same sense; the Roman Epicureans had intermundia (neuter plural) for "spaces between the worlds" (translating Greek metakosmia).
interplay (n.): 1838, from inter- "between" + play (n.). "Reciprocal play," thus "free interaction."
Interpol: 1952, contraction of international police (in full, The International Criminal Police Commission), founded 1923 with headquarters in Paris.
interpolate (v.): 1610s, "to alter or enlarge (a writing) by inserting new material," from Latin interpolatus, past participle of interpolare "alter, freshen up, polish;" of writing, "falsify," from inter- "among, between" (see inter-) + polare, which is related to polire "to smoothe, polish."

Sense evolved in Latin from "refurbish," to "alter appearance of," to "falsify (especially by adding new material)." Middle English had interpolen (early 15c.) in a similar sense. Related: Interpolated; interpolating.
interpolation (n.): 1610s, "act of interpolating;" 1670s, "that which is interpolated," from French interpolation (17c.) or directly from Latin interpolationem (nominative interpolatio), noun of action from past participle stem of interpolare "to alter; falsify" (see interpolate).
interpolator (n.): 1650s, from Late Latin interpolator "one who corrupts or spoils," agent noun from past participle stem of Latin interpolare "to polish; to alter; to falsify" (see interpolate).
interpose (v.): 1590s (trans.); 1610s (intrans.), from Middle French interposer (14c.), from inter- "between" (see inter-) + poser "to place" (see pose (v.1)). Related: Interposed; interposing.
interposition (n.): late 14c., from Old French interposicion "interpolation, intercalation; suspension, break" (12c.), from Latin interpositionem (nominative interpositio) "an insertion," noun of action from past participle stem of interponere "to put between, place among; put forward," from inter- "between" (see inter-) + ponere "to put, place" (past participle positus; see position (n.)).
interpret (v.): late 14c., "expound the meaning of, render clear or explicit," from Old French interpreter "explain; translate" (13c.) and directly from Latin interpretari "explain, expound, understand," from interpres "agent, translator," from inter- "between" (see inter-) + second element of uncertain origin, perhaps related to Sanskrit prath- "to spread abroad," PIE *per- (5) "to traffic in, sell" (see pornography). Related: Interpreted; interpreting.
interpretable (adj.): 1610s, from Late Latin interpretabilis "that can be explained or translated," from Latin interpretari "explain, expound, understand" (see interpret).
interpretation (n.): mid-14c. "a translated text, a translation" (late 13c. in Anglo-French), from Old French interpretacion, entrepretatiun "explanation, translation" (12c.) and directly from Latin interpretationem (nominative interpretatio) "explanation, exposition," noun of action from past participle stem of interpretari "explain, expound; understand" (see interpret).

From late 14c. as "act or process of explaining or interpreting; an explanation; construction placed upon an action." Meaning "dramatic or musical representation" is from 1880.
interpretative (adj.): "meant to explain," 1560s, from past participle stem of Latin interpretari "explain, expound; understand" (see interpret). Interpretive means the same but is considered to be less correctly formed, because -ive adjectives are normally formed on the Latin past participle. Related: Interpretatively.
interpreter (n.): "one who translates spoken languages; a translator of written texts," late 14c., from Old French interpreteor, entrepreteur, from Late Latin interpretator "an explainer," agent noun from interpretari "explain, expound" (see interpret).
interpretive (adj.): 1670s, from interpret + -ive, perhaps on model of assertive or other like words, where the -t- belongs to the Latin stem. The preferred formation is interpretative. Listed by Fowler among the words "that for one reason or another should not have been brought into existence."
interpunction (n.): "punctuation, a point inserted in writing," 1610s, from Latin interpunctionem (nominative interpunctio) "a putting of points between (words), division by points," noun of action from past participle stem of interpungere "to put points between," from inter- "between" (see inter-) + pungere "to prick, pierce," related to pugnus "a fist" (see pugnacious).
interracial (adj.): also inter-racial, "existing or taking place between races," 1883, from inter- "between" + racial (adj.).
interregnum (n.): 1580s, from Latin interregnum "an interval between two reigns," literally "between-reign," from inter- "between" (see inter-) + regnum "kingship, dominion, rule, realm," related to regere "to rule, to direct, keep straight, guide" (see regal). In the republic, it meant a vacancy in the consulate. The earlier English noun was interreign (1530s), from French interrègne (14c.).
interrelate (v.): also inter-relate, 1831 (implied in interrelated), transitive, "bring into reciprocal relation," from inter- "between" + relate (v.). Intransitive sense "come into reciprocal relation" is attested from 1912. Related: Interrelating.
interrelation (n.): 1841, from inter- "between" + relation.
interrelationship (n.): also inter-relationship, "state of being interrelated," 1841, from inter- "between" + relationship.
interrogate (v.): late 15c., a back-formation from interrogation or else from Latin interrogatus, past participle of interrogare "to ask, question." The Old French word was interroger (14c.) which yielded English interroge (late 15c.), now obsolete. Related: Interrogated; interrogating.
interrogation (n.): late 14c., "a question;" c. 1500, "a questioning; a set of questions," from Old French interrogacion "a questioning" (13c.) or directly from Latin interrogationem (nominative interrogatio) "a question; questioning; judicial inquiry," noun of action from past participle stem of interrogare "to ask, question, inquire; interrogate judicially, cross-examine," from inter- "between" (see inter-) + rogare "to ask, to question" (see rogation).
interrogative (adj.): "asking or denoting a question," c. 1500, from Late Latin interrogativus "pertaining to a question," from interrogat-, past participle stem of Latin interrogare "to ask, question, inquire; interrogate judicially, cross-examine," from inter- "between" (see inter-) + rogare "to ask, to question" (see rogation). As a noun, "word implying a question," 1520s. Related: Interrogatively.
interrogator (n.): 1751, from French interrogateur (16c.) or directly from Late Latin interrogator, agent noun from Latin interrogare "to ask, question" (see interrogation).