Reify: "To convert (an abstraction or mental construction) into a supposed real thing; to attribute substantiality to; to hypostatize (to assume as a reality). ~Webster's

A word describing a concrete, fixed definition or idea of something that is an indefinably complex conception.
Climate, war, politics, the public, and God are examples of reified concepts.
...notice that each person develops their own concrete, fixed definition (or image) for each of these complex concepts; although we all use the same word to refer to any given complex concept.