Idea: Mental representation of something.

Note that ideas may also represent falsehoods and impossibilities. Ideas represent fantasies, which may be real if they are not false or impossible. Also, one may have ideas one cannot now, at least, put into words. An example is the tip of the tongue phenomenon: One knows one knows a term for something, but cannot now recall it.

As defined one needs not to be conscious of ideas that one has, and indeed it makes much sense to assume that one may represent many things one is not conscious of that one represents. If one has a conscious idea, i.e. an idea one is conscious of, it makes sense to call it a concept.

Note that human beings can have quite positive ideas about non-existing or impossible things, as witnessed (!) by mermaids or what M.C. Escher's drawings depict.

Other noteworthy human ideas - apart from a deus absconditus - are the infinities of infinities opened up or suggested by Cantorian set theory; the square root of minus 1 that enters complex numbers; or n-dimemsional geometries with n>3.

In brief: The human mind is able to imagine more and other than there is or can be, and reason rationally about these entities as well.