Yeah, Jeff's pretty much got it right...
It's kinda like porn; I know it when I see it.
I'd define "rare" in terms of how hard it is to get the card for my collection, money and condition notwithstanding. If a card shows up once every few years, that's rare. If I can buy a specimen tomorrow if I want to complete a set, that's common. Everything else is degrees. The T206 Wags and Plank [common back] are not rare; the error Doyle and Cobb/Cobb are. I can't afford any of them but if I had the cash I'd have a Wagner or a Plank, but I'd probably be sitting around waiting for the others to show up.
I don't consider "condition rarities" to be rare, for reasons I've blathered on about repeatedly in the past: subjectivity establishing the 'grade', lack of true population vs. graded numbers, etc.