1. Something, such as a thought or conception, that potentially or actually exists in the mind as a product of mental activity.
2. An opinion, conviction, or principle: has some strange political ideas.
3. A plan, scheme, or method.
4. The gist of a specific situation; significance: The idea is to finish the project under budget.
5. A notion; a fancy.
6. Music A theme or motif.
7. Philosophy
   1. In the philosophy of Plato, an archetype of which a corresponding being in phenomenal reality is an imperfect replica.
   2. In the philosophy of Kant, a concept of reason that is transcendent but nonempirical.
   3. In the philosophy of Hegel, absolute truth; the complete and ultimate product of reason.
8. Obsolete A mental image of something remembered.

Synonyms: These nouns refer to what is formed or represented in the mind as the product of mental activity. Idea has the widest range: "Human history is in essence a history of ideas" (H.G. Wells).
Thought is distinctively intellectual and stresses contemplation and reasoning: "Language is the dress of thought" (Samuel Johnson).
Notion often refers to a vague, general, or even fanciful idea: "She certainly has some notion of drawing" (Rudyard Kipling).
Concept and conception are applied to mental formulations on a broad scale: You seem to have absolutely no concept of time. "Every succeeding scientific discovery makes greater nonsense of old-time conceptions of sovereignty" (Anthony Eden).