Here's what I'm using these days as I get better organized. Perhaps availability scale might be a better term.

Common - You can always buy one
Uncommon - Are available at most major shows, and come up regularly for sale
Scarce - 2-5 specimens are typically sold each year
Rare - One specimen every one to five years
Very Rare - Less than rare
< 10 known - Speaks for itself

There are some notes and coins I've needed recently that fall somewhere between Uncommon and Scarce according to your scale. Think of something that you can't find at major shows most of the time. Maybe 1 out of 4 or 5 major shows a year. But that's still more common than scarce because there may be a few dozen major shows across the country and others may have examples.

With ancients it seems like between the auctions which are online, and vcoins, you see a very large percentage of the available stock virtually. I know with medals it's a lot more coin show or ebay, so it's harder to know how many show up in a given year. Of course, any scale is somewhat arbitrary. I just thought for my own sake I'd verbalize what I mean when I use these terms.