I think "mid-fi" is mostly a marketing term (as opposed to a sonic descriptive), usually understood to mean a brand (as opposed to a piece of gear) that is intended to bridge the presumed gap in sound, design, appearance and price between mass-market gear and hi-end audiophile gear. As a marketing, pricing, engineering and styling approach, mid-fi has been typified by brands such as NAD, Rotel, Adcom and Parasound. Whether this hierarchy of brand and price actually correlates directly with sound quality -- especially at the lower and higher ends of the mid-fi price scale, where there can be considerable overlap with the higher and lower ends of the mass-market and high-end price scales, respectively -- is another question, one for which the answer is probably often taken for granted. Although I've used the term mid-fi myself in these forums, one problem I can see with it is that it could be taken to imply that hi-end necessarily equals hi-fi (high fidelity) and mass-market necessarily equals lo-fi, both very questionable assumptions.
