the sound, the way they last through the years, the way they present them self(s). the weave of the guitars. the vocie(s), the melody,the way it frees you or takes you places. thats how i would define classic rock.
The fact that no matter how many years have passed, they have never gone out of style. Their music is as fresh now as it was then. It's wonderful to see young people turn on to the Beatles or other groups from the sixties.
A lot of classic rock purist say ''classic rock'' was an era. 1965 to 1975.
I agree, but personally I'd expand the era to 1964 to the end of the 70's. And I'd through in the first 5 years of the 80's. After that rock music slowly went downhill, with a lot of hits and misses, over the last 20-25 years.
So, in my opinion, any rock music that was recorded between 1964 and 1985 can be called, ''classic rock''.