Online Etymology Dictionary

interamnian (adj.): "between two rivers" (usually, if not exclusively, with reference to Mesopotamia), 1774, from Late Latin interamnius, from inter "between" (see inter-) + amnis "a river," a word perhaps of Celtic origin (see afanc).
interblend (v.): "intermingle," 1823, from inter- + blend (v.). Related: Interblended; interblending.
interbreed (v.): 1803 (transitive) "to breed by crossing species or varieties," from inter- + breed (v.). Intransitive sense of "procreate with one of a different species" is from 1825. Related: Interbred; interbreeding.
intercalary (adj.): "inserted into the calendar," 1610s, from Latin intercalarius "intercalary, of an intercalary month," from intercalare "proclaim an intercalary day" (see intercalate). General sense of "interpolated" is attested from 1798.
intercalate (v.): "to insert a day into the calendar," 1610s, from Latin intercalatus, past participle of intercalare "to proclaim the insertion of an intercalary day," from inter- "between" (see inter-) + calare "to call" (an intercalary day; see calendar). Related: Intercalated; intercalating.

A necessary process in the Roman calendar to balance the solar and lunar aspects of it. Intercalation was done after Feb. 23 or 24 (the terminalia), every two or four years. Twenty-seven days were intercalated, making a full intercalary month (which included the last four or five days of Februarius), known as mensis intercalaris (and also known, according to Plutarch, as Mercedonius). No one now knows why the intercalation was done in the middle of February rather than after its end, unless it was because the important festivals at the end of that month (Regifugium and Equirra) were closely associated with holidays in early March. After Caesar's reform (46 B.C.E.) the only intercalary day is Feb. 29 every four years.
intercalation (n.): 1570s, from Latin intercalationem (nominative intercalatio) "insertion of an intercalary day," noun of action from past participle stem of intercalare "proclaim an intercalary day" (see intercalate). The general sense "insertion of any addition into an existing series" is from 1640s.
intercede (v.): 1570s, "to come between in space or time" (obsolete); c. 1600, "to interpose on behalf of another," a back-formation from intercession, or else from Latin intercedere "intervene, come between, be between" (in Medieval Latin "to interpose on someone's behalf"), from inter- "between" (see inter-) + cedere "go" (see cede). Related: Interceded; interceding.
intercept (v.): c. 1400, "to cut off" (a line), "prevent" (the spread of a disease), from Latin interceptus, past participle of intercipere "take or seize between, to seize in passing," from inter- "between" (see inter-) + -cipere, comb. form of capere "to take, catch" (see capable). Related: Intercepted; intercepting.
intercept (n.): "that which is intercepted," from intercept (v.). From 1821 of a ball thrown in a sport; 1880 in navigation; 1942 in reference to secret messages.
interception (n.): early 15c., "action of intercepting" (the flow of a bodily fluid), from Latin interceptionem (nominative interceptio) "a seizing, taking away," noun of action from past participle stem of intercipere (see intercept (v.)). Specific football/rugby sense is attested by 1897. Meaning "action of closing in on and destroying an enemy aircraft, etc." is recorded from 1939.
interceptor (n.): 1590s, from Latin interceptor "interceptor, usurper, embezzler," agent noun from intercipere (see intercept (v.)). As a type of fast fighter aircraft, from 1930. Intercepter is attested from c. 1600.
intercession (n.): early 15c., "act of interceding;" c. 1500, "intercessory prayer, a pleading on behalf of oneself or another," from Latin intercessionem (nominative intercessio) "a going between, coming between, mediation," noun of action from past participle stem of intercedere "intervene, come between, be between" (in Medieval Latin "to interpose on someone's behalf;" see intercede). The sense "pleading on behalf of another" developed in Christianity.
intercessor (n.): "one who pleads or intervenes on behalf of another," late 15c., from a specific Christian use of Latin intercessor "one who intervenes, a mediator," agent noun from intercedere (see intercede). Related: Intercessory.
interchange (v.): late 14c., enterchaungen, "to give and receive reciprocally; to alternate, put each in place of the other" (trans.), also "change reciprocally" (intrans.), from Old French entrechangier "interchange, exchange," from entre- "between" (see inter-) + changier "to change" (see change (v.)). Related: Interchanged; interchanging.
interchange (n.): early 15c., "an exchange, act of exchanging reciprocally," from Old French entrechange, from entrechangier (see interchange (v.)). Meaning "alternate succession" is from 1550s. In reference to a type of road junction, 1944.
interchangeability (n.): 1763; see interchangeable + -ity.
interchangeable (adj.): late 14c., entrechaungeable, "mutual, reciprocal," from inter- + changeable. Meaning "capable of being used in place of each other" is from 1560s. Related: Interchangeably.
intercoastal (adj.): "within the coasts," 1927, from inter- + coastal.
intercollegiate (adj.): 1873, from inter- + collegiate.
intercom (n.): "radio or telephone intercommunication system," 1937, colloquial shortening of intercommunication, which is attested from 1911 in reference to systems of linked telephones.
intercommunicate (v.): 1580s, "communicate reciprocally," from inter- + communicate (v.) or else from Medieval Latin intercommunicatus, past participle of intercommunicare.
intercommunication (n.): mid-15c., "discussion, conference," from Anglo-Latin intercommunicationem; see inter- + communication). Attested from 1881 in reference to systems of linked telephones.
intercommunion (n.): 1749, "intimate intercourse, fellowship," from inter- "between" + communion (n.).
interconnect (v.): 1863, from inter- + connect (v.). Related: Interconnected; interconnecting.
interconnectedness (n.): 1873, noun of state from past participle of interconnect + -ness. Interconnection is attested from 1827.
intercontinental (adj.): 1825, American English, from inter- "between" + continental (adj.). Of missiles, from 1956.
intercostal (adj.): "between the ribs," 1590s; see inter- + costal.
intercourse (n.): mid-15c., "communication to and fro," ("In early use exclusively with reference to trade" [OED]), from Old French entrecors "exchange, commerce, communication" (12c., Modern French entrecours), from Late Latin intercursus "a running between, intervention," in Medieval Latin "intercommunication," from intercursus, past participle of intercurrere "to run between, intervene, mediate," from Latin inter- "between" (see inter-) + currere "to run" (see current (adj.)).

Sense of "frequent and habitual meeting and contact, social communication between persons" is from 1540s. Meaning "mental or spiritual exchange or intercommunication" is from 1560s. Meaning "sexual relations" (1798) probably is a shortening of euphemistic sexual intercourse (1771) with intercourse in its sense "social contact and relations."
intercrural (adj.): "between the thighs," or in medicine, "between leg-like structures," 1690s, from inter- "between" + Latin crus "shin, shank, (lower) leg; supports of a bridge," from Proto-Italic *krus-, which is of uncertain origin.
interdepartmental (adj.): also inter-departmental, 1861, from inter- + departmental.
interdependence (n.): 1816 (Coleridge), from inter- + dependence.
interdependency (n.): 1830, from interdependent + -cy.
interdependent (adj.): 1817 (Coleridge), from inter- + dependent. Related: Interdependently.
interdict (v.): c. 1300, enterditen, "to place under ban of the Church, excommunicate," from Old French entredit (Modern French interdit), past participle of entredire "forbid by decree, excommunicate," from Latin interdicere "interpose by speech, prohibit, forbid," from inter- "between" (see inter-) + dicere "to speak, to say" (see diction). General sense "forbid, prohibit" in English is from early 15c. Related: Interdicted; interdicting; interdictory.
interdiction (n.): mid-15c., enterdiccioun, from Latin interdictionem (nominative interdictio) "a prohibiting, a forbidding," noun of action from past participle stem of interdicere (see interdict).
interdisciplinary (adj.): 1937, from inter- + disciplinary.
interest (n.): mid-15c., "legal claim or right; a concern; a benefit, advantage, a being concerned or affected (advantageously)," from Old French interest "damage, loss, harm" (Modern French intérêt), from noun use of Latin interest "it is of importance, it makes a difference," third person singular present of interresse "to concern, make a difference, be of importance," literally "to be between," from inter- "between" (see inter-) + esse "to be" (see essence). The sense development to "profit, advantage" in French and English is not entirely clear.

The earlier Middle English word was interesse (late 14c.), from Anglo-French interesse "what one has a legal concern in," from Medieval Latin interesse "compensation for loss," noun use of Latin interresse (compare German Interesse, from the same Medieval Latin source).

Financial sense of "money paid for the use of money lent" (1520s) earlier was distinguished from usury (illegal under Church law) by being in reference to "compensation due from a defaulting debtor." Sense of "personal or selfish consideration" is from 1620s. Meaning "business in which several people are interested" is from 1670s. Meaning "curiosity, feeling that something concerns one, appreciative or sympathetic regard" is first attested 1771. Interest group is attested from 1907; interest rate by 1868.
interest (v.): "cause to be interested, engage the attention of," c. 1600, earlier interesse (1560s), from the noun (see interest (n.)). Perhaps also from or influenced by interess'd, past participle of interesse.
interested (adj.): 1660s, "characterized by concern or sympathy," past-participle adjective from interest (v.). From 1828 as "having an interest or stake (in something);" sense "motivated by self-interest" (1705) is perhaps a back-formation from disinterested. Related: Interestedness.
interesting (adj.): 1711, "that concerns, important" (archaic), present-participle adjective from interest (v.). Meaning "engaging the attention, so as to excite interest" is from 1751. Related: Interestingly. Euphemistic phrase interesting condition, etc., "pregnant" is from 1748.
interface (n.): 1874, "a plane surface regarded as the common boundary of two bodies," from inter- + face (n.). Modern use is perhaps a c. 1960 re-coinage; McLuhan used it in the sense "place of interaction between two systems" (1962) and the computer sense "apparatus to connect two devices" is from 1964. As a verb from 1967. Related: Interfaced; interfacing.
interfacial (adj.): 1837, of crystals, from inter- "between" + facial (adj.).
interfaith (adj.): 1921, from inter- + faith.
interfere (v.): formerly also enterfere, mid-15c., "to strike against," from Middle French enterferir "exchange blows, strike each other," from entre- "between" (see entre-) + ferir "to strike," from Latin ferire "to knock, strike," related to Latin forare "to bore, pierce" (see bore (v.), and compare punch (v.), which has both the senses "to hit" and "to make a hole in").

Figurative sense of "to meddle with, oppose unrightfully" is from 1630s. Related: Interfered; interfering. Modern French interférer is from English.
interference (n.): 1783, "intermeddling," from interfere on model of difference, etc. In physics, in reference to the mutual action of waves on each other, from 1802, coined in this sense by English scientist Dr. Thomas Young (1773-1829). Telephoning (later broadcasting) sense is from 1887. In chess from 1913; in U.S. football from 1894.
interferometer (n.): "instrument for measuring the interference of light waves," 1897, a hybrid from interfere + -meter. Compare interferential (1867), coined on the model of differential. Related: Interferometric; interferometry.
interferon (n.): animal protein, 1957, coined in English from interfere + subatomic particle suffix -on; so called because it "interferes" with the reduplication of viruses.
interflow (n.): "a flowing into each other," 1839, from inter- + flow (n.).
interfold (v.): 1570s, from inter- + fold (v.). Related: Interfolded; interfolding.
intergalactic (adj.): 1928, in reference to galaxies as presently understood, from inter- + galactic. The word itself was in use by 1901, when galaxies were thought to be a sort of nebulae.