idea

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

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 ).

C16: via Late Latin from Greek: model, pattern, notion, from idein to see

It is usually considered correct to say that someone has the idea of doing something, rather than the idea to do it: he had the idea of taking (not the idea to take) a short holiday