i dont know why so many people have jammed this up with BS answers. its VERY SIMPLE. there is NO AMBIGUITY to its meaning.
an incredibly transparent, revealing system with the highest fidelity that acts as a standard for comparison (reference) for all other audio systems and media.
in other words, the standard is so high that it its fidelity is more like a calibrated measuring device than a not.
the phrase "reference system" can be used however one wants, but that doesnt mean its correct.
What is a "reference" system?
OK, most of us have an audio system, mine even works some of the time. But I see that some members have "reference" systems. This has to be better than the assorted junk that I have piled together. Probably a lot better than your rig as well.
But really, what distinguishes these folks "reference" systems from the pedestrian systems that the rest of us have? There must be something, or they could just save the keyboard time and drop the word when discussing their gear. And I am not referring to reviewers, that is a different story and one to examine more carefully in the future.