All these words denote something that exists in the mind during the processes of perceiving, thinking or willing. Idea is the most general term, applicable to almost any part or aspect of mental activity.

• The idea of death is frightening to most people; I had no idea of going to the party; She has a general idea of the cost of the repair job.

A concept is an idea of a category or kind that has been generalized from particular instances. Thus, the concept of "cat" arises from the many cats we see. Concept also refers to a widely held idea of what something is or should be: The concept of government of many small nations has been influenced by the legislative and judicial institutions of Britain. The meaning of conception is much like this second sense of concept , but differs in that the idea of what a thing is or should be is here held by an individual or small group and is often coloured by imagination and feeling: A child’s conception of the universe is formed by his limited experience and his own fancies. A thought is an idea based on intellectual activity, and is not directly attributable to a sense impression: He has many thoughts on the matter. A notion is a vague or capricious idea , often without any sound basis: to have a notion that the share market will rise. An impression is an idea that arises from something external. It suggests either a half-formed mental picture or a superficial view or conclusion.

• An infant may have the impression that its father is a giant; My impression is that he was lying.