idea

1400–50; < Late Latin < Greek idéā form, pattern, equivalent to ide- (stem of ideîn to see) + -ā feminine noun ending; replacing late Middle English idee < Middle French < Late Latin, as above; akin to wit¹

1, 2. Idea, thought, conception, notion refer to a product of mental activity. Idea, although it may refer to thoughts of any degree of seriousness or triviality, is commonly used for mental concepts considered more important or elaborate: We pondered the idea of the fourth dimension. The idea of his arrival frightened me. Thought, which reflects its primary emphasis on the mental process, may denote any concept except the more weighty and elaborate ones: I welcomed his thoughts on the subject. A thought came to him. Conception suggests a thought that seems complete, individual, recent, or somewhat intricate: The architect's conception delighted them. Notion suggests a fleeting, vague, or imperfect thought: a bare notion of how to proceed.

English idea comes from one of Seneca’s Epistles (58), written about a.d. 64 during his retirement from Emperor Nero’s court, in which the Roman philosopher uses idea in the sense of “Platonic idea, eternal archetype.” Seneca wrote idea in Latin letters; the Roman orator Cicero, about a hundred years earlier, wrote the same word, with the same meaning, but in Greek letters. Plato used the perfectly ordinary Greek noun idéa “form, shape” as a term in logic meaning “classification, principle of classification,” and in his own metaphysics to mean “ideal form, prototype.” In fact, the earliest uses of idea in English show semantic overlap with ideal. The familiar and current meanings having to do with a mental conception, notion, or image first appeared in the late 16th century.
The Greek noun idéa comes from the very common, very complicated Proto-Indo-European root weid-, woid-, wid- “to see.” In Greek the variant woid- forms the verb oîda ( woîda in some dialects), meaning “I know.” (In form, oîda is a perfect tense used to show a present state: “I have seen, I know.”)
Woidos, a noun derived from woid-, becomes veda- “knowledge” in Sanskrit ( Rig-Veda means “knowledge of the hymns, sacred stanzas”). The variant wid- forms the Greek noun idéa, and the infinitive ideîn (also wideîn ), the Latin infinitive vidēre, and the Slavic (Czech) vidět, all meaning “to see.”
Weid-, woid-, wid- become wīt-, wait-, wit- in Germanic. The suffixed form wīt-to- forms the adjective wīsaz, Old English wīs (English wise ), and Old English wīsdōm “learning” (English wisdom ).