Expanding on my earlier post, I must respectfully disagree with those who seem to be using the three terms to distinguish between different kinds of audiophile-oriented equipment, particularly on the basis of price.
As even a casual perusal of these forums will show, there is lots of equipment priced well under $1K that can provide respectable audiophile-calibre sound. Referring to that equipment as "low fi" strikes me, frankly, as absurd.
Along the lines of my earlier comment, it seems to me that the three terms should be used to distinguish audiophile-oriented equipment, often made by specialist manufacturers, from mass market-oriented equipment, usually made by large corporations. The latter falling into one of the two lower categories, and the former being, at least ostensibly, "hifi."
Certainly there are hifi/high-end products that are sonic failures, and can be described as producing mid-fi sound (or worse), but without a frame of reference for those terms that is broader than the audiophile market, what would describing their sound as mid-fi even mean?
Regards,
-- Al