What defines mid-fi versus high-end?

"High End" refers only to price.

"Mid Fi" refers to the current system of all audiophiles.

'Mid-Fi' is an arbitrary marketing term thrown about loosely by those with something to gain by creating artificial barriers to entry. And painting products, even entire brands with that brush, they hope to justify there own products and brands exorbitant pricing. Case in point is Marantz. Their products span pricing from $500 to $10,000. A $500 receiver is of necessity a cost engineered product; a $9,000 integrated  amp, decidedly less so, and clearly a 'high-end' product. Yet despite impeccable technical execution, I have heard it derided as 'too mid-fi'. That is simple snobbery with some unacceptable implied overtones.

Does it matter what you or others call it? If you are entertained by and enjoy it...

The key is to get the best sound reproduction for YOU within your budget.

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.