idea

n.

1. Something one thinks or knows

   concept, conception, thought, notion, impression, inkling, construct; see also opinion 1, thought 2.

2. A conviction

   opinion, view, doctrine, conception; see belief 1, faith 2.

3. A plan

   intention, design, scheme, approach; see plan 2, purpose 1.

4. Fancy

   whimsy, whim, fantasy; see fancy 3.

5. Meaning

   sense, import, purport; see meaning.

idea, the most general of these terms, may be applied to anything existing in the mind as an object of knowledge or thought; concept refers to a generalized idea of a class of objects, based on knowledge of particular instances of the class his concept of a republic; conception, often equivalent to concept, specifically refers to something conceived in the mind, or imagined my conception of how the role should be played; thought is used of any idea, whether or not expressed, that occurs to the mind in reasoning or contemplation she rarely speaks her thoughts; notion suggests a vague thought or one not fully considered I had a notion to go; impression also implies vagueness of an idea provoked by some external stimulus I have the impression that she's unhappy