This list includes 88 Hard words that start with I, from “iatrogenic” to “isthmus”. Many entries are polysyllabic, technical, or archaic and useful in academic writing, tests, and editorial work.

Hard words that start with I are challenging, low-frequency vocabulary items drawn from medicine, law, science, and literature. For example, “iatrogenic” reminds readers of medicine while “isthmus” evokes geography and classical roots.

Below you’ll find the table with Etymology and Definition.

Etymology: Shows the word’s origin, language, and root elements so you understand historical meaning and modern usage.

Definition: One-sentence definition (part of speech plus clear sense) that helps you use the word correctly.

Hard words that start with I

WordPart of speechMeaningEtymology
ibidemadv.In the same place; used to repeat a prior citation.Latin ibīdem “in the same place”, Classical Latin, c.1st century BC.
ichorn.Mythic bloodlike fluid of the gods; a thin discharge.Greek ikhor “divine fluid”, Classical Greek, antiquity.
ichnographyn.A ground plan or floor plan; planimetric drawing.Greek ichnos “track” + graphein “to write”, 17th century.
ichthyologyn.The scientific study of fishes.Greek ichthys “fish” + -logia, New Latin/Modern, 18th century.
ichthyosisn.A hereditary skin condition causing dry, scaly skin.Greek ichthys “fish” + -osis, medical coinage, 19th century.
ichneumonn.A parasitic wasp or a mongoose-like tracker animal.Greek ikhneúmōn “tracker”, Classical Greek, 17th century in English.
iconoclastn.Person who attacks or rejects cherished beliefs or institutions.Greek eikon “image” + klastes “breaker”, 16th century.
iconographyn.Visual images or symbolic representations in art.Greek eikon “image” + graphein “to write”, 17th century.
iconostasisn.A screen or wall of icons in Eastern Orthodox churches.Greek eikon “image” + stasis “standing”, Late Greek/Church usage.
idempotentadj.Unchanged by repeated application, especially in math/computing.Latin idem “same” + potent, mathematical coinage, 19th century.
idiosyncrasyn.A distinctive or peculiar habit, characteristic, or trait.Greek idiosynkrasia “personal temperament”, Hellenistic Greek, 17th century.
idolatryn.Worship of idols or excessive devotion to something.Greek eidōlolatria “idol worship”, Late Latin/Old French, medieval usage.
ileumn.The final section of the small intestine.Latin īleum from Greek eileo, anatomical term, Classical antiquity.
ileitisn.Inflammation of the ileum.New Latin ileum + -itis, medical coinage, 19th century.
illimitableadj.Without limits; boundless or infinite.Latin in- “not” + limitare, Medieval/Modern Latin, 16th century.
imbroglion.A complicated, embarrassing, or confusing situation.Italian imbroglio “tangle”, from broglio, 17th century.
imbuev.To saturate or inspire thoroughly with a quality.Latin imbuere “to wet, dye”, Classical Latin, 16th century.
imbricateadj./v.Overlapping like roof tiles or scales; to overlap.Latin imbricatus “tiled”, from imbrex, Classical Latin, 17th century.
immanentadj.Present within and inherent to something; indwelling.Latin immanere “to remain in”, Late Latin, philosophical use, 17th century.
immaterialadj.Not relevant or lacking physical substance.Latin immaterialis “not material”, Medieval Latin, 16th century.
immemorialadj.So long ago as to be beyond memory or record.Latin immemorabilis/immemor, Medieval Latin, Middle English usage.
immolatev.To sacrifice, often by burning; to destroy as offering.Latin immolare “to sacrifice on an altar”, Classical Latin, 16th century.
immiscibleadj.Unable to mix to form a homogeneous mixture (liquids).Latin in- “not” + miscēre “to mix”, chemical usage, 19th century.
immunogenicadj.Capable of inducing an immune response.Modern Latin immuno- + -genic, medical coinage, 20th century.
impecuniousadj.Having little or no money; penniless.Latin impecuniosus “without money”, from pecunia, 16th century.
impignoratev.To pledge or pawn something as security.Latin impignorare “to pledge”, Medieval Latin, 15th century.
imprecationn.A spoken curse or invocation of misfortune on someone.Latin imprecari “to pray against”, Classical Latin, 15th–16th century.
imprimaturn.Official authorization to publish, often ecclesiastical.Latin imprimatur “let it be printed”, ecclesiastical Latin, 16th century.
impugnv.To call into question or challenge as false.Latin impugnare “to attack”, Classical Latin, 14th–15th century.
imputev.To attribute an action or characteristic to someone.Latin imputare “to reckon to”, Medieval Latin, 14th century.
inchoateadj.Just begun; undeveloped or rudimentary.Latin incohere/inchoare “to begin”, Medieval Latin, 16th century.
incendiaryadj./n.Causing fire or provoking strong feelings; arsonist device.Latin incendium “blaze”, Classical Latin, 16th century.
incipientadj.In an initial stage; beginning to exist or appear.Latin incipere “to begin”, Classical Latin, 17th century usage.
incunabulan.Books or prints produced before ca. 1501; early printings.Latin incunabula “swaddling clothes; beginnings”, bibliographic term.
indefatigableadj.Unable to be tired; persistently tireless.Latin indefatigabilis “not to be wearied”, 17th century.
indefeasibleadj.Not able to be annulled, voided, or undone.Latin in- “not” + defeasibilis, legal term, 17th century.
indelibleadj.Impossible to remove, erase, or forget.Latin indelebilis “not to be blotted out”, 16th century.
indolentadj.Habitually lazy; causing little or no pain (medical).Latin indolens “not feeling pain”, Classical Latin, 16th century.
induratev./adj.To harden physically or emotionally; hardened.Latin indurare “to harden”, Classical Latin, 15th century.
ineffableadj.Too great or extreme to be expressed in words.Latin ineffabilis “unutterable”, Medieval Latin, 14th century.
inefficaciousadj.Not producing the desired effect; ineffective.Latin in- “not” + efficax, Medieval Latin, 17th century.
inexorableadj.Impossible to stop or persuade; relentless.Latin inexorabilis “not to be entreated”, Medieval Latin, 14th century.
infinitesimaladj.Extremely small; immeasurably minute.Latin infinitesimus “infinitely small”, mathematical coinage, 17th century.
infinituden.The quality or state of being infinite; vastness.Latin infinitudo, Medieval/Modern Latin, 17th century.
ingeniousadj.Marked by inventive skill or cleverness.Latin ingeniosus “having talent”, Classical Latin, 16th century.
ingenuousadj.Open, honest, and unsuspecting; naive in a charming way.Latin ingenuus “native, frank”, Classical Latin, 16th century.
inkhornadj./n.Pertaining to pedantic, overly learned language; archaic term.From inkhorn, Early Modern English, 16th–17th century.
inimicaladj.Hostile; harmful or adverse to someone’s interests.Latin inimicus “enemy”, Classical Latin, 16th century.
iniquitousadj.Grossly unjust or morally wrong.Latin iniquitus “unequal, unjust”, Medieval Latin, 15th century.
insalubriousadj.Unhealthy or unwholesome, especially of climate or habits.Latin in- “not” + salubris “healthy”, 17th century.
insidiousadj.Proceeding subtly and harmfully; treacherous in a slow way.Latin insidiosus “deceitful, ambushing”, Classical Latin, 17th century.
insipientadj.Lacking knowledge, judgment, or sense; foolish.Latin insipiens “not wise”, Medieval Latin, 16th century.
insouciantadj.Carefree; casually indifferent or unconcerned.French insouciant “not worried”, from in- + souci, 19th century.
insouciancen.A relaxed and unconcerned attitude; nonchalance.French insouciance, 19th century.
insuperableadj.Impossible to overcome or surmount.Latin in- “not” + superabilis, Medieval Latin, 15th century.
intercalatev.To insert a day into a calendar or insert between layers.Latin intercalare “to insert a day”, Roman practice, Classical Latin.
interlocutorn.Someone who takes part in a dialogue or conversation.Latin interlocutor “one who speaks between”, Medieval Latin, 17th century.
interlopev.To intrude or trespass into a place or activity unwarranted.Middle English interlopen “to run between”, 16th century.
internecineadj.Mutually destructive within a group or between rivals.Latin internecīnus “mutually deadly”, Medieval Latin, 16th century.
interregnumn.An interval between reigns, administrations, or governments.Latin interregnum “between kings”, Classical Latin, 16th century.
intersticen.A small intervening space or gap between things.Latin interstitium “that which stands between”, Medieval Latin, 15th century.
intimationn.A subtle or indirect hint or suggestion.Latin intimare “to make known”, Medieval Latin, 16th century.
intractableadj.Difficult to manage, control, or cure.Latin in- “not” + tractabilis, Medieval Latin, 17th century.
intransigentadj.Refusing to compromise; uncompromising in position.Spanish/Latin intransigens, political coinage, 19th century.
intumescentadj.Swelling as by internal pressure; becoming enlarged.Latin intumescere “to swell up”, Classical Latin, 19th century technical usage.
intricacyn.Complexity due to many interrelated parts; detailed complexity.Latin intricare “to entangle”, Medieval Latin, 17th century.
inundatev.To flood or overwhelm with water or large quantities.Latin inundare “to overflow”, Classical Latin, 15th century.
invectiven.Abusive or strongly critical language; denunciation.Latin invectiva “vehement speech”, Medieval Latin, 15th century.
invidiousadj.Likely to arouse resentment or cause ill will.Latin invidiosus “full of envy”, Classical Latin, 15th century.
inveterateadj.Firmly established by long persistence; habitual.Latin inveteratus “grown old”, Classical Latin, 16th century.
inviolateadj.Free from violation, injury, or desecration; intact.Latin inviolatus “not violated”, Medieval Latin, 16th century.
iridescentadj.Showing luminous rainbow colors that change with angle.Latin iris “rainbow”, from Greek, Modern coinage, 19th century.
iridologyn.Study claiming iris patterns indicate health; considered pseudoscience.Greek iris “iris” + -logia, 20th-century coinage.
irredentismn.Political movement to reclaim lost territory for national reasons.Italian irredento “unredeemed”, 19th century political term.
irremediableadj.Impossible to remedy or cure; irreversible.Latin in- “not” + remediabilis, Medieval Latin, 16th century.
irradiancen.Energy from radiation arriving per unit area; radiative flux.Latin irradiāre “to irradiate”, physics term, 19th–20th century.
isagogicadj.Serving as an introduction, especially to a study or text.Greek eisagōgē “introduction”, Late Latin/Greek, 17th–19th century.
isobaricadj.Having equal pressure; relating to constant-pressure processes.Greek isos “equal” + baros “weight, pressure”, 19th-century science.
isobarn.A line on a map connecting points of equal atmospheric pressure.Greek isos “equal” + baros “weight”, meteorological term, 19th century.
isochronousadj.Occurring at equal time intervals; synchronous in period.Greek isos “equal” + chronos “time”, 19th-century physics/musical term.
isoglossn.A geographic boundary between linguistic features or dialects.Greek isos “equal” + glossa “tongue”, linguistic term, 19th century.
isomorphicadj.Having similar form or structure; corresponding in pattern.Greek isos “equal” + morphe “form”, mathematics/biology, 19th century.
isotropicadj.Having identical properties in all directions.Greek isos “equal” + tropos “turn”, physics/materials term, 19th century.
isthmusn.A narrow strip of land connecting two larger areas, with water sides.Greek isthmos “neck”, Classical Greek, ancient usage.
iatrogenicadj.Caused by medical examination or treatment.Greek iatros “physician” + -genic “produced by”, medical coinage, 19th century.
ideologuen.A person who zealously advocates an ideology, sometimes uncritically.French idéologue, early 19th century political coinage.
idoneousadj.Suitable or appropriate; adequate (archaic/rare).Latin idoneus “suitable”, Classical Latin, Early Modern English.
illativeadj.Serving as a logical inference or resulting conclusion.Latin illatus “brought in”, philosophical/grammatical term, 17th century.

Descriptions

ibidem
Scholarly citation term meaning “as cited above”; common in footnotes and bibliographies.
ichor
Poetic and mythological term; also used figuratively for a pale or unhealthy discharge.
ichnography
Technical in architecture and archaeology for precise plan drawings; rare outside specialist contexts.
ichthyology
A branch of zoology; useful term for biology students and naturalists.
ichthyosis
Medical term; appears in dermatology contexts describing fish-scale like skin.
ichneumon
Used in entomology (wasps) and classical contexts for a tracking creature.
iconoclast
Often applied to radicals in religion, art, or politics who challenge traditions.
iconography
Used in art history to analyze themes, symbols, and recurring motifs.
iconostasis
Specific liturgical term; important in studies of Eastern Christian art and ritual.
idempotent
Technical in algebra and computer science; idempotent operations yield same result on repetition.
idiosyncrasy
Useful for describing quirky behaviors or unique stylistic features.
idolatry
Used both religiously and metaphorically for extreme reverence.
ileum
Common in anatomy; students should distinguish ileum from ilium (pelvis).
ileitis
Medical diagnosis term often discussed in gastroenterology.
illimitable
Poetic and emphatic; used for abstract or grand scales.
imbroglio
Colorful loanword often used about political or social scandals.
imbue
Common in literary contexts: to imbue a work with mood or theme.
imbricate
Used in geology, biology, and design to describe layered patterns.
immanent
Philosophical and theological term contrasted with “transcendent.”
immaterial
Can mean “unimportant” in law or “nonphysical” in metaphysics.
immemorial
Often in fixed phrases like “time immemorial” in legal or historic contexts.
immolate
Historic/ritual term; used metaphorically to mean ruin or sacrifice.
immiscible
Common in chemistry to describe oil and water-like behavior.
immunogenic
Used in immunology and vaccine development contexts.
impecunious
Literary and formal term for poverty; appears in older literature and exams.
impignorate
Rare legal/archaic verb meaning to mortgage or pawn an item.
imprecation
Occurs in literature and religious contexts describing curses.
imprimatur
Historically used by censors; now figurative for official approval.
impugn
Common in formal writing: to impugn evidence or motives.
impute
Legal and analytic term: impute responsibility or blame.
inchoate
Frequently appears in law and literary criticism for nascent states.
incendiary
Used literally for fire-starting and figuratively for provocative rhetoric.
incipient
Useful descriptor in science, business, and criticism.
incunabula
Specialist bibliographic term prized by rare-book collectors and historians.
indefatigable
High-register compliment for sustained effort or stamina.
indefeasible
Occurs in law and property contexts describing absolute rights.
indelible
Used for permanent marks or unforgettable impressions.
indolent
Both literary (lazy) and medical senses (painless conditions).
indurate
Medical and figurative uses: indurate tissue or an indurate heart.
ineffable
Common in religious and poetic contexts for sublime experiences.
inefficacious
Formal term in medicine, law, and policy critique.
inexorable
Often used for fate, time, or unyielding processes.
infinitesimal
Important in calculus and physics; used metaphorically for tiny amounts.
infinitude
Poetic and philosophical term emphasizing unboundedness.
ingenious
Often mispaired with “ingenuous”—means inventive rather than naive.
ingenuous
Useful contrast to “dissembling”; common on advanced vocab lists.
inkhorn
Literary criticism term for pompous or needless jargon.
inimical
Formal adjective often used for conditions or relationships that oppose goals.
iniquitous
High-register moral term seen in literature and sermons.
insalubrious
Used in public health, travel writing, and urban description.
insidious
Often applied to disease, propaganda, or plots that work covertly.
insipient
Archaic/rare; appears in literary criticism for poor judgment.
insouciant
Stylish loanword for breezy nonchalance in personal demeanor.
insouciance
Used in cultural commentary and character description.
insuperable
Applied to obstacles, problems, or challenges deemed overwhelming.
intercalate
Technical in calendrics and geology (layer insertion) with literal sense.
interlocutor
Formal term for a debater, conversation partner, or Q&A participant.
interlope
Often used of traders or people who meddle in others’ affairs.
internecine
Used of civil wars, feuds, or conflicts harming both sides.
interregnum
Used historically and metaphorically for leadership gaps.
interstice
Poetic and technical term in architecture and anatomy.
intimation
Useful in literary analysis and polite discourse.
intractable
Applies to problems, patients, or personalities resistant to change.
intransigent
Describes hardline actors in negotiations or ideology.
intumescent
Used in medicine and materials science to describe swelling behavior.
intricacy
Describes elaborate plots, mechanisms, or designs.
inundate
Used literally for floods and figuratively for surges of work or mail.
invective
Common in literary criticism for harsh verbal attacks.
invidious
Often used about comparisons or actions that offend fairness.
inveterate
Describes entrenched habits or long-standing attitudes.
inviolate
Used for sanctified places, treaties, or moral principles.
iridescent
Describes oils, shells, or materials with shimmering color shifts.
iridology
Mentioned in alternative medicine discussions; regarded skeptically by scientists.
irredentism
Used in history and political science to explain territorial revanchism.
irremediable
Strong word for permanent harms or losses.
irradiance
Technical in optics and solar energy; used in engineering contexts.
isagogic
Rare academic term for introductory remarks or frameworks.
isobaric
Used in meteorology and thermodynamics.
isobar
Common in weather maps and atmospheric studies.
isochronous
Used in engineering, music, and clockwork to mean regular timing.
isogloss
Important in dialectology for mapping language variation.
isomorphic
Technical in math (structure-preserving map) and general analogy.
isotropic
Key concept in physics, materials science, and crystallography.
isthmus
Geographical term seen in history and geography texts.
iatrogenic
Important in medicine: side effects or harms caused by caregivers.
ideologue
Often used critically in political analysis and journalism.
idoneous
Rare legal/literary adjective for fitness or suitability.
illative
Used in logic and rhetoric concerning inferential moves.
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