Since what you are looking for is a definition, there is no one answer. Someone like Mr. Webster has to define the term and get it accepted by the world.

To me, Obsolete means a chip that is "no longer used in a casino for its original purpose".

This means chips that can be redeemed but no longer played (A.C.), are obsolete. It also covers the No Cash Value chip situation; these were never redeemable anyway, so they become obsolete when the casino no longer will allow their use on a table game, or in a tournament if that was the intended purpose.

It's useful to have another term, like Retired to cover some of the other situations like the A.C. casinos.

In my definition, Obsolete is not necessarily a "forever" term. If a casino should begin using a chip again that collectors formerly considered Obsolete, then it's a live chip again. Remember, a few places will accept for play or redemption any chip that they ever issued for play. Caesars Palace is one example.