Idiot. From Ancient Greek ἰδιώτης, idiōtēs, a private citizen, one who has no professional knowledge. From ἴδιος, idios, one's own, pertaining to oneself, private. Iδιώτης, idiōtēs, was used in ancient Athens to refer to one who declined to take part in public life.
Derived or related words: idiotic, idiotically.
Idiom. From Ancient Greek ἰδίωμα, idioma, a peculiarity, property, a peculiar phraseology. As the above, from ἴδιος, idios, one's own, pertaining to oneself, private.
Derived or related words: idiomatic, idiomatically.
Idea. From Ancient Greek ἰδέα, idea, notion, pattern. From εἴδω, eidō, I see. Also from the word idea derives the word ideal, optimal, being the best possibility and ideology, the body of beliefs or principles belonging to an individual or group.
Derived or related words: ideal, ideology, idealism, idealize, ideologue.
Idol. From Ancient Greek εἴδωλον, eidōlon, image, idol. From εἶδος, eidos, form, image, shape. An image or representation of anything that is believed to convey spiritual power. A cultural icon, or especially popular person.
Derived or related words: idolize, idolatry.
Idiosyncrasy. From Ancient Greek ἰδιοσυγκρασία, idiosunkrasia, one’s own temperament. From ἴδιος, idios, one’s own + σύν, sin, together + κρᾶσις, krasis, temperament. A behaviour or way of thinking or language that is particular of a person or a group.
Derived or related words: idiosyncratic, idiosyncratically.
Irony. From Ancient Greek εἰρωνεία, eirōneia. From εἴρων, eirōn, one who feigns ignorance. A statement that, when taken in context, may actually mean something different from, or the opposite of what is written literally; the use of words expressing something other than their literal intention, notably as a form of humor.
Derived or related words: ironic, ironically.