It helps to think about it relatively. We evaluate positions with the following notation: White is winning (+-), White is ahead (+/-), White is slightly better (+/=), The position is even (=), Black is slightly better (=/+), Black is ahead (-/+), Black is winning (-+). I usually think of a mistake as a move that makes the evaluation of the position one group lower and blunders, two or more groups lower....
Mistakes:
White is slightly better out of the opening and makes a middlegame move that allows Black to equalize.
Black is better because of an attack, but misses a move to stop White's counterplay.
Most of us feel what blunders are but it's worth pointing out that blunders don't have to result in a losing position. If you squander a winning attack down to a draw, it still qualifies as playing blunderfully.
"Mistakes were sold out, so I had to order all the blunders. The difference is priceless." - a poor Master of nonsense
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