I guess it is not a really sharply defined term, but I mean some fundamental piece of the universe that is really out there as opposed to something that emerges from something more fundamental or is just a convenient model or abstraction. As I said in a other comment something like temperature which is just a nice tool to deal with the momenta of many particles where the motion of those particles is more fundamental. Not the best analogy as pointed out by another commenter but I think it gets the point across.