Frankly, it's whatever you say it is. For some people, it could be the $1 million dollar system at AXPONA that featured the new $395k Wilson Audio XVX speakers. For someone else, it could be an NAD 3020 driving a pair of Dayton Audio Classic C65 speakers that cost $75 a pair.
To me, the concept of a "reference" system is just another aspect of "audiophilia nervosa."