idea (n.) Look up idea at Dictionary.com
late 14c., "archetype of a thing in the mind of God; Platonic `idea,'" from Latin idea "idea," and in Platonic philosophy "archetype," from Greek idea "ideal prototype," literally "the look of a thing (as opposed to the reality); form; kind, sort, nature," from idein "to see," from PIE *wid-es-ya-, suffixed form of root *weid- "to see" (see vision). Sense of "result of thinking" first recorded 1640s.
Men of one idea, like a hen with one chicken, and that a duckling. [Thoreau, "Walden"]
Idée fixe (1836) is from French, literally "fixed idea."