The conception of a standard or principle to be realized or aimed at; a conception of what is desirable or ought to be; a governing conception or…

What should be.

That which exists only as a mental conception, especially an idealized one; an imaginary, usually idealized, thing.

Something which exists only as a mental conception, something ideal or imaginary; an idealized conception.

An instance of idealizing; an act or product of idealizing; an ideal representation.