1, 2. Idea, thought, conception, notion refer to a product of mental activity. Idea, although it may refer to thoughts of any degree of seriousness or triviality, is commonly used for mental concepts considered more important or elaborate: We pondered the idea of the fourth dimension. The idea of his arrival frightened me. Thought, which reflects its primary emphasis on the mental process, may denote any concept except the more weighty and elaborate ones: I welcomed his thoughts on the subject. A thought came to him. Conception suggests a thought that seems complete, individual, recent, or somewhat intricate: The architect's conception delighted them. Notion suggests a fleeting, vague, or imperfect thought: a bare notion of how to proceed. 4. sentiment, judgment.