For me mid-range has always meant a bike which sits at least three or four models from the top of a larger brands' range.
Groupset: Mainly LX, STX, XC-E or lower but possibly with a rear mech or perhaps shifters up-specced to make it more appealing to the potential buyer.
Components: Mainly built with manufacturer branded items like the stem, bars etc, as opposed to non OEM parts.
As for frame material I'd say 4130 steel, or 7000 series alloy, but not ti or 6061 etc.
And it had to be available as a complete bike for about £600 or less.
That's me for starters...
I reckon that's pretty bang on in my opinion, especially the point about frame material. Most mid range bikes were offered to give the most balanced package of frame and components that wouldn't fall to bits if they were ridden seriously off road, or even raced a little.. Therefore boutique frames with cheaper bits don't really qualify
Several bikes, one supportive lady and still trying not to buy anything else!