In most collecting circles mint is perfect. No flaws, marks, scratches, and basically the way it came from the manufacturer assuming there were no defects. Then there could be excellent, very fine, fine, very good, good, fair and poor. I would NOT use all of these on pins. (I certainly would not want to be the one to come up with all the criteria for each classification)To me in most collecting circles this takes the fun out of collecting. Not to say I don't want my pins in good condition, I do. Take a look at how action figures (and I think baseball cards) are graded C-9 and down. I do not want to become so obbsessed with pins to the point that I am scouring over them with a magnifying glass. I want this to be fun. If a hobby isn't fun then what is the point?
BTW if you are refering to an auction I would say that in some cases the seller is using the term because he/she there is no formal way to grade pins. Good, excellent and mint may mean the same thing, that the pin is new, on it's card, maybe still in a bag if that is how it came, and has no serious/noticable flaws.
Pins are hand made and some small differences are bound to creep in.