you'll end up with a million answers to that question. personally there's accurate, competition accurate, hunting accurate, and "good nuff fer critters"
I personally consider a rifle accurate if I can shoot under an inch group regularly at 100 yards.
I consider a rifle extremely accurate if it can regularly shoot under 1 inch, but I'm not exactly capable of that control so I've never had an extremely accurate rifle.
then there's hunting accurate, generally this is a gun that I wouldn't use for long range application, but as long as I can hold a 2 inch group at 100 yards, that's a 4 inch group at 200 and is usually the maximum distance I'll shoot when hunting.
critter accurate is basically 4 inch at 100 yards and is only really effective at close range for varmints and the like.
then you also have to consider the difference between precision and accuracy. precision is the ability to shoot the same area repeatedly, accuracy is the ability to hit the general area that you want it to, you can be precise by shooting a dime 5 times, but that dime can be 2 feet to the left of your target and still be considered precision shooting. likewise you can shoot the same piece of paper that you were aiming at, but none make it on the bullseye, that's accurate but without a high degree of precision.