Depends on the individual’s ear as well, as everyone has different preferences as well as levels of expertise on how to listen. But if my way were the hifi-way, hifi simply means holographic. Natural and detailed in presentation to the point that it is believable that the musicians are in the room playing for you, and the images are completely vivid and communicate everything they are supposed to for you. Anything less is less than hifi.
The most important thing here is that the room and setup have as much to do with the sound as the gear does. For example, I’ve heard high-priced gear that does not sound hifi whatsoever because of poor setup or synergy.
Mid-fi to me is gear that is a gateway to hifi sound but is unable to image or resolve to the level of fidelity I describe above.