What defines mid-fi versus high-end?

Answer: your income.  What you can afford is seen inevitably as the "low-end" of audiophilia.  The highest end is always to dream for, whether $5,000 or $500,000.  Does really high quality sound demand that?  No, not at all.  But human nature does.

I thought I was omniscient, but frankly, this one has me stumped. 

There are different levels if HI-FI.  I guess it is difficult question. Everyone's ears are different, tastes are different. I would define HI Fi as being pure sounding music with a full bodied spectrum of sound and dynamics. Not a bose wave radio though they sound good.  After you reach a certain dollar level you have to pay out much more $$$$$$$ to reach the next level. You get to the point where for me I get very happy and do not need to upgrade for a few decades. Spending money on HI Fi is not a hobby for me. Enjoying a great sounding system is a joy in life and a blessing.

If it is sold at the Best Buy, then it is probably mid fi.

Yep sure.