The real answer is that there is no "true", government approved, official definition.
For example, this year, I think that one of the ski mags (Powder?) classified the 10ex and g4 as mid-fats, even though they have tips around 116 mm and waists around 83 mm.
From the way these skis handle, I kinda agree with that classification, however, most other people probably still feel that the upper limit to mid-fats is around 75-78 mm (waist), and hence skis like the 10ex and g4 should be called lite-fats (or something similar).
The lower boundary to real "fats" is generally considered to be around 90 mm (waist), and some people have informally termed skis over 100 mm (waist) as "super-fats".
As Oboe pointed out earlier, the lower boundary to the mid-fat range is pretty well agreed upon to be in the 68-70 mm (waist) range.
Tom / PM