Classic rock. We all use the term, but we also all have different definitions of it. Case in point: once when I was in the middle of my college radio program, a visiting high school student came into the station and started talking to me. He asked me what the greatest classic rock band was that still toured together.
"Easy," I answered, "The Rolling Stones."
"Nope. Guess again," he said. I made several more guesses before he got frustrated and told me it was Pearl Jam.
Aside from making me feel like I was 90, I also figured out what the confusion over the term is. Some people, like the kid in my anecdote, use the term as we would say that something is a classic. As in, that band is great and will be forever. But for other people, "classic rock" is an era--when rock was really good and not this modern noise polluting the airwaves. This definition of the genre tends to shift with the changing times. Hence, when I was in high school, classic rock stations tended to play great stuff from the 60s and 70s, but today they mostly play hair metal. And there is also a third group that uses classical to mean "in the style of" historically-based classic rock.
This all tends to get confusing, so I propose that we do away with the term "classic," and instead think of rock like we do classical music. Rock is the genre, which we can divide into historical periods and place its various movements and figures into them based on common sound, concerns, and chronology.
Early Period: roughly 1900-1956. All the folk-based popular music genres from which rock and roll grew out of, such as blues, bluegrass, honky-tonk, rhythm and blues, and so on. Rockabilly, as a transitional genre, falls into this period and the next.
Baroque Period: roughly 1954-1965. The sound we recognize as rock and roll emerges. Elvis, Buddy Holly, Chuck Berry, Little Richard, Carl Perkins, etc. Doo-wop and girl groups. Early Beatles and Beach Boys. We might even include some early Motown in this period.
Classical Period: roughly 1965-1975. The rock and roll sound becomes codified. Well, as codified as a genre that celebrates rule breaking can be. Rock and roll here matures into rock music. The British invasion, folk rock, blues-based hard rock. Later Motown like Marvin Gaye's What's Going On? The Rolling Stones, The Who, Led Zeppelin.
Romantic Period: roughly 1968-1985. Musicians take the "sound" of rock and go crazy with it. Psychedelic rock, singer-songwriter, prog rock (the Wagner of rock), funk, heavy metal. Pink Floyd, Cat Stevens, Metallica.
Post-modern Period: roughly 1972-present. (Note: I've skipped modernism because, as a 20th Century artform, all of rock is of the modernist period.) Questions the rules of rock and assumptions we make about it, but also shares characteristics of post-modern movements in other media. Glam rock (identity politics), punk (post-structuralism), hip-hop (collage), roots rock (nostalgia). Bowie, Sex Pistols, Public Enemy, the Black Crows. Also, any "revival" of a particular sound is post-modern in a sort-of neoclassical way.
So, Pearl Jam, coming out of the grunge movement, which fuses punk simplicity with heavy metal power, is clearly post-modern, and not "classic" rock. Take that, kid!