What Is Classic Rock?

I was flipping around the radio channel and when I got to the classic rock station, they were playing some late 90s hair band and it immediately made me think "hey, that's not classic rock." But then I started thinking about the referent of that vague term. Oldies is fairly well-defined -- from doo-wop up to but not including the Beatles. After oldies is classic rock which then would be Beatles and Stones up to...?

It could be like "oldies" a definite transition. Punk and grunge were reactions to the corporatized nature of rock and roll that came out of the baby boom generation, so we could draw the line at anything released post-Sex Pistols. But a new album by Tom Petty, Van Morrison, or Bruce Springsteen still seems to be classic rock even though it is contemporary. We could say, anyone who started making music before the Pistols.

It could be a portable line like the way we determine when a car is an antique. Any rock older than 25 years is deemed classic. In this way, we constantly accumulate music in the category. The problem here seems to be that it would mean that there would be no identifiable essential properties that makes something classic rock. Would They Might Be Giants or XTC be classic rock? Would we then include punk? That doesn't seem right either.

So, what do we mean by classic rock and how do we determine what belongs in and what belongs out of the category?