| infallible | (adjective) incapable of failure or error | - |
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| infamous | (adjective) known widely and usually unfavorably | Synonyms: ill-famed, notorious |
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| infantile | (adjective) of or relating to infants or infancy | - |
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| (adjective) indicating a lack of maturity | Synonyms: childish |
| (adjective) being or befitting or characteristic of an infant | - |
| infatuated | (adjective) marked by foolish or unreasoning fondness | Synonyms: enamored, in love, potty, smitten, soft on, taken with |
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| infeasible | (adjective) not capable of being carried out or put into practice | Synonyms: impracticable, unfeasible, unworkable |
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| infected | (adjective) containing or resulting from disease-causing organisms | Synonyms: septic |
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| infectious | (adjective) easily spread | - |
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| (adjective) of or relating to infection | - |
| (adjective) caused by infection or capable of causing infection | Synonyms: infective |
| infective | (adjective) caused by infection or capable of causing infection | Synonyms: infectious |
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| (adjective) able to cause disease | Synonyms: morbific, pathogenic |
| infelicitous | (adjective) not appropriate in application; defective | - |
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| (adjective) marked by or producing unhappiness | Synonyms: unhappy |
| inferential | (adjective) relating to or having the nature of illation or inference | Synonyms: illative |
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| (adjective) derived or capable of being derived by inference | - |
| (adjective) based on interpretation; not directly expressed | - |
| (adjective) of reasoning; proceeding from general premisses to a necessary and specific conclusion | - |
| (adjective) resembling or dependent on or arrived at by inference | Synonyms: illative |
| inferior | (adjective) written or printed below and to one side of another character | Synonyms: subscript |
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| (adjective) of or characteristic of low rank or importance | - |
| (adjective) of low or inferior quality | - |
| (adjective) having an orbit between the sun and the Earth's orbit | - |
| (adjective) lower than a given reference point | - |
| (adjective) falling short of some prescribed norm | Synonyms: deficient, substandard |
| infernal | (adjective) being of the underworld | - |
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| (adjective) characteristic of or resembling Hell | - |
| (adjective) of or pertaining to or characteristic of a very uncontrolled and intense fire | - |
| (adjective) expletives used informally as intensifiers | Synonyms: blame, blamed, blasted, blessed, damn, damned, darned, deuced, goddam, goddamn, goddamned |
| (adjective) extremely evil or cruel; expressive of cruelty or befitting hell | Synonyms: demonic, diabolic, diabolical, fiendish, hellish, satanic, unholy |
| infertile | (adjective) incapable of reproducing | Synonyms: sterile, unfertile |
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| infinite | (adjective) having no limits or boundaries in time or space or extent or magnitude | - |
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| (adjective) (of verbs) not having tense, person, or number (as a participle or gerund or infinitive) | Synonyms: non-finite, nonfinite |
| (adjective) total and all-embracing | - |
| (adjective) too numerous to be counted | Synonyms: countless, innumerable, innumerous, multitudinous, myriad, numberless, uncounted, unnumberable, unnumbered, unnumerable |
| infinitesimal | (adjective) infinitely or immeasurably small | Synonyms: minute |
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| infinitival | (adjective) relating to or formed with the infinitive | - |
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| infirm | (adjective) lacking bodily or muscular strength or vitality | Synonyms: debile, decrepit, feeble, rickety, sapless, weak, weakly |
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| (adjective) lacking firmness of will or character or purpose | - |
| inflamed | (adjective) adorned with tongues of flame | - |
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| (adjective) lighted with red light as if with flames | Synonyms: ablaze, reddened |
| (adjective) resulting from inflammation; hot and swollen and reddened | - |
| inflammable | (adjective) easily ignited | Synonyms: flammable |
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| inflammatory | (adjective) arousing to action or rebellion | Synonyms: incendiary, incitive, instigative, rabble-rousing, seditious |
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| (adjective) characterized or caused by inflammation | - |
| inflatable | (adjective) designed to be filled with air or gas | - |
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| inflated | (adjective) pretentious (especially with regard to language or ideals) | Synonyms: high-flown, high-sounding |
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| (adjective) enlarged beyond truth or reasonableness | Synonyms: hyperbolic |
| inflationary | (adjective) associated with or tending to cause increases in inflation | - |
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| inflected | (adjective) (of the voice) altered in tone or pitch | - |
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| (adjective) showing alteration in form (especially by the addition of affixes) | - |
| inflectional | (adjective) characterized by inflections indicating grammatical distinctions | - |
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| inflexible | (adjective) resistant to being bent | - |
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| (adjective) incapable of change | - |
| (adjective) not making concessions | Synonyms: sturdy, uncompromising |
| (adjective) incapable of adapting or changing to meet circumstances | Synonyms: rigid, unbending |
| inflowing | (adjective) flowing inward | Synonyms: influent |
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| influent | (adjective) flowing inward | Synonyms: inflowing |
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| influential | (adjective) having or exercising influence or power | - |
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| informal | (adjective) not formal | - |
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| (adjective) used of spoken and written language | - |
| (adjective) having or fostering a warm or friendly and informal atmosphere | Synonyms: cosy, cozy, intimate |
| (adjective) not officially recognized or controlled | Synonyms: loose |
| informational | (adjective) relating to or having the nature of information | - |
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| informative | (adjective) providing or conveying information | Synonyms: informatory |
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| (adjective) tending to increase knowledge or dissipate ignorance | Synonyms: enlightening, illuminating |
| (adjective) serving to instruct or enlighten or inform | Synonyms: instructive |
| informatory | (adjective) providing or conveying information | Synonyms: informative |
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| informed | (adjective) having much knowledge or education | - |
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| infrahuman | (adjective) belonging to a group below humans in evolutionary development | - |
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| inframaxillary | (adjective) relating to the lower jaw | Synonyms: mandibular |
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| infrangible | (adjective) not capable of being violated or infringed | Synonyms: absolute, inviolable |
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| (adjective) difficult or impossible to break or separate into parts | - |
| infrared | (adjective) having or employing wavelengths longer than light but shorter than radio waves; lying outside the visible spectrum at its red end | - |
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| infrasonic | (adjective) having frequencies below those of audible sound | - |
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| infrequent | (adjective) not frequent; not occurring regularly or at short intervals | - |
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| infuriated | (adjective) marked by extreme anger | Synonyms: angered, enraged, furious, maddened |
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| infuriating | (adjective) extremely annoying or displeasing | Synonyms: exasperating, maddening, vexing |
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