This is an interesting debate, but one that I think is ultimately doomed not to be resolved!
There have been a lot of good suggestions, but in the final analysis what prevents us from defining the term absolutely is lack of reliable information. The mere fact that a chip doesn't appear on tables doesn't mean that it's necessarily obsolete. (Example: I happened to be in Atlantic City a few years ago when the "obsolete" Bally's first issue $1 with dark brown edge inserts was put back out on the tables. A $40 chip became a $3 chip overnight!)
Many casinos won't sell chips from the cashier's cage. Does that mean that if you walk up to three tables and don't see it, it's obsolete? No, especially not if the edges of the chips look just the same as the current chips from that casino - they may well be buried in stacks on the tables. (Another example: I can recall picking up two "obsolete" and fairly high-priced Union Plaza bicentennials for face value at a craps table a few years ago, simply because they found their way to the top of a stack and I happened to ask the dealers for them.
One of the problems here is that we're dealing with several issues at once. What is a "limited edition"? How many is "limited"? We have no definition of that either, and so long as casinos can order more "limited editions" whenever they want to, we'll never be able to define the term. (Another example: when CHIPCO made the $25 25th anniversary chip for Caesars Palace a few years ago, there were articles in newspapers and coin publications around the country announcing the chip and indicating that of the limited number made - which I don't recall offhand - they would be available and in use at the casino only during the anniversary year, after which all remaining would be destroyed. Yet there has not been one word that I've been able to find, to indicate that a single chip was ever destroyed.)
Another issue we're dealing with (although we've skirted it in these discussions) is far more basic, I think. What is a "chip"? And more importantly, what is a "CASINO chip"? A "chip" can be bloody near anything, can't it? It doesn't necessarily have to have anything to do with a casino to be called a "chip." (Example: the Las Vegas Sports Card Connection's advertising pieces that some casino chip collectors have.)
And what's a "casino chip"? Anything issued in conjunction with a casino? (Example, the Mizpah Hotel high values of a few years ago.) Anything issued by a casino, whether for gaming purposes or not? (Example: the Hard Rock parking lot chips sold in sets.) Anything that says "casino" on it, but which has absolutely nothing to do with public gaming? (Example, Paul-Son's Casino de Mexico chips, sold for home use.)
And, as another thread discusses, what's a "limited edition"? How many is "limited"? And what happens when casinos can reorder "limited" editions?
The more you dig into these questions, the more difficult accurate and inclusive and acceptable definitions become. I seriously doubt that our hobby will be able to come up with generally-acceptable definitions, and I further doubt that the CC>CC Board wants to get into the business of policing advertising or descriptions. And we certainly have little or no influence over casinos and what they do. Despite repeated attempts, the Nevada Gaming Control Board has refused to meet with serious chip collectors to discuss these kinds of issues. Ultimately, aside from the fact that we chip collectors represent a source of income, the gaming industry couldn't care less!