Dic: 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).

... The American Heritage