idea, concept, conception, thought, notion, impression mean what exists in the mind as a representation (as of something comprehended) or as a formulation (as of a plan).

idea may apply to a mental image or formulation of something seen or known or imagined, to a pure abstraction, or to something assumed or vaguely sensed.

concept may apply to the idea formed by consideration of instances of a species or genus or, more broadly, to any idea of what a thing ought to be.

conception is often interchangeable with concept; it may stress the process of imagining or formulating rather than the result.

thought is likely to suggest the result of reflecting, reasoning, or meditating rather than of imagining.

notion suggests an idea not much resolved by analysis or reflection and may suggest the capricious or accidental.

impression applies to an idea or notion resulting immediately from some stimulation of the senses.