idea, 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 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 .