We have quite a list going on here, although I have to disagree with Angels and Airwaves. At current, I consider classic alternative anything that was considered alternative prior to Nirvana's "Smells Like Teen Spirit," which debuted on Billboard's Modern Rock Tracks chart on 9/21/1991. I'm not saying this date is THE definitive line, but sort of a benchmark. Established bands like U2, R.E.M., Duran Duran, The Cure, and The B-52s still had releases going into '92 or '93 that could easily fit into the classic alternative mold because it's not grungy sounding. However, kind of like how the definition of "classic rock" has changed over time, "classic alternative" could easily incorporate the grunge (Nirvana, AIC, Pearl Jam) and funk/psychedelic (RHCP, Jane's Addiction) music that was popular from '91 to '95. I lived through the explosion of grunge music and feel that there should be some kind of dividing line at around 1991. The Sirius XM alternative station, for the most part, have it right in their programming of those channels (First Wave, Lithium), although I think they tend to leave out the folkier alternative tracks (Edie Brickell, Indigo Girls) and Bob Marley & the Wailers has always been a progressive/alternative staple, but doesn't get recognized on 1st Wave either, although I know they're getting played elsewhere.
Jacko