idea, concept, notion

Any thought existing in the mind may be called an idea, concept, or notion. The most widely used of these words, idea, should be applied to thoughts that are serious or elaborate: "The surgeons weighed the idea of an immediate operation." "The idea of armed conflict is frightening." A concept (or conception) suggests a thought that is complete, detailed, and even intricate: "The artist's concept of portrait painting is highly unorthodox." A notion is a fleeting, vague, hastily formed, or imperfect thought: "I had no notion this is what you were planning." Recommendation: use notion for an idea that has not been pondered and weighed; use idea or concept for important thoughts that have been given serious attention. Occasionally try substituting for idea (the generally used term) such words as opinion, belief, view, conviction, theory, hypothesis, or whim. Any word that you use may be as vague as idea, but you will avoid overworking a threadbare term.