intricacy, n. complication, entanglement, complexity.

intricate, a. complicated, entangled, involved, labyrinthian, mazy.

intrigue, n. conspiracy, machination, cabal, plot; amour (illicit love affair), liaison.

intrigue, v. plot, scheme, machinate, cabal.

intriguer, n. machinator, plotter, schemer, Figaro, intrigant, intrigante (female).

intriguing, a. scheming, Machiavelian, wily.

intrinsic, a. inherent, innate, real, genuine.  Antonym:  extrinsic.

introduction, n. preface, proem, exordium, prelude, prolegomenon, prologue, foreword; presentation; interpolation interposition, initiation.

introductory, a. preliminary, proemial, prefatory.

intrude, v. infringe, invade, encroach, obtrude, interlope.

intrusion, n. encroachment, obtrusion, infringement, interloping.

intrusive, a. obtrusive.  Antonyms:  unintrusive, unobtrusive, retiring.

intrust, v. commit, confide.

invade, v. infringe, encroach, trespass, intrench, trench.

invalid, a. null, void, of no efficacy; baseless, unfounded.

invalid, a. valetudinary.

invalid, n. valetudinarian.

invalidate, v. annul, abrogate, cancel, quash, nullify.

invalidation, n. nullification, abrogation, annulment.

invalidism, n. valetudinarianism.

invasion, n. irruption, foray, raid, aggression; violation, infringement, encroachment, intrenchment.

inveigle, v. wheedle, seduce.

inveiglement, n. wheedling, seduction.

investigate, v. probe, sift, examine.

investigation, n. examination, inquiry, research.

invidious, a. objectionable, offensive.

invincible, a. indomitable, unconquerable.

invisible, a. imperceptible, microscopic, evanescent.

invitation, n. bidding, call, summons.

invite, v. bid, ask, summon; allure, induce, entice.

invocation, n. prayer, supplication.

involuntary, a. uncontrollable, instinctive, automatic, compulsory, forced.

inwards, n. pl. entrails, intestines, bowels, guts; viscera; giblets, inmeats, umbles.

iota, n. scintilla, tittle, atom, mite, jot, whit, bit, particle.

Ireland, n.  Erin, Emerald Isle.

iridescent, a. prismatic, nacreous.

iris, n. flower-de-luce, fleur-de-lis, flag.

Irish, a.  Hibernian.

Irishman, n.  Hibernian, Milesian, Teague, Pat, Paddy.

iron, v. smooth; mangle.

irons, n. pl. fetters, chains, handcuffs, shackles, manacles, hampers, gyves.

ironworker’s pneumonia. siderosis.

irrecoverable, a. irretrievable.

irregular, a. inconformable, abnormal, anomalous, exceptional, unsymmetrical, unusual, erose, baroque, aberrant.

irregularity, n. inconformableness, abnormality, asymmetry, anomaly, aberration, intermittency.