According to The Free Online Dictionary, here is a comparison of synonyms of idea:
Synonyms: idea, thought, notion, concept, conception
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).