Obviously it is a very gray area, and a lot of people will consider my definition of "Hi Fi" to be too generous, but as a very general guideline I've always thought of these terms as follows:
Lo Fi: Compact "all-in-one" systems, characterized by lots of features, low quality, and low price.
Mid Fi: Systems that are typically built around a receiver, combining a preamplifier, amplifier, and tuner in one component, that is typically manufactured by relatively large companies such as Sony, Onkyo, Denon, Yamaha, etc.
Hi Fi: Components made by specialist manufacturers, with the emphasis on quality rather than features. The kind of stuff that is typically seen here at A'gon.
Regards,
-- Al