【Idea】 , 【concept】 ,  【conception】 ,  【thought】 ,  【notion】 ,  【impression】  mean what exists in the mind as a representation of something that it apprehends or comprehends or as a formulation of an opinion, a plan, or a design.

【Idea】 is the most comprehensive and widely applicable of these terms; it may be used of an image of something at one time or another actually perceived through the senses, or of something never perceived but visualized from bits of information or of something that is the clearly or vaguely defined product of fancy, imagination, or inventive power.

It may denote a mere supposition or a good or practical solution or suggestion or a ridiculous or preposterous suggestion.

【Concept】 applies in logic to the 【idea】 of a thing which the mind conceives after knowing many instances of the category to which it belongs and which is devoid of all details except those that are typical or generic. In more general use the term applies to a formulated and widely accepted 【idea】 of what a thing should be.

【Conception】  is often used in place of  【concept】  in this latter sense; in fact it is sometimes preferred by those who wish to keep  【concept】  as a technical term of logic. However  【conception】 so strongly suggests the activity of the mental power of bringing into existence an 【idea】 of something not yet realized or not yet given outward form that it often implies not only the exercise of the reflective powers but of the imagination as colored by feeling; the term therefore more often applies to a peculiar or an individual 【idea】 than to one held by men as a whole or by an entire class, profession, or group.

【Conception】 is also, especially in literary and art criticism, the usual term for the 【idea】 or design conceived by the writer or artist in advance of or in company with his giving it expression or form.

【Thought】 applies either to an expressed or to an unexpressed 【idea】 , especially one that comes into the mind as a result of meditation, reasoning, or contemplation.

【Notion】  often adds to  【idea】 ’s implication of vagueness the suggestion of caprice or whim or of half-formed or tentative purpose or intention, but  【notion】  may also come close to  【concept】 in suggesting a general or universal 【concept】 or to  【conception】 in denoting the meaning content assigned by the mind to a term.

【Impression】 (see also  【IMPRESSION】 1 ) usually suggests an 【idea】 which comes into the mind as the result of an external stimulus.