noun

1. something one thinks, knows, or imagines; a thought; mental conception or image; notion
2. an opinion or belief
3. a plan; scheme; project; intention; aim
4. a hazy perception; vague impression; fanciful notion; inkling
5. meaning or significance
6. Music a theme or figure
7. Philos. according to Plato, any of the unchanging, eternal, intelligible models or archetypes of which all material things are only imperfect imitations and from which their existence derives: in modern philosophy, used variously to mean the immediate object of thought, absolute truth, etc.

Etymology: L < Gr, form or appearance of a thing as opposed to its reality < IE *widswo- < base *weid-, to see, know > L videre, to see, Gr idein, to see, OE witan, to know