[He paused thoughtfully wondering how to put it into words exactly.]
Well...I think it's kind of an outdated term now really. It was used a lot in the 90's for bands that didn't really sound like anything else that was out at the time? That's what it always seemed like it would mean to me anyways.
When I think of alternative I think of stuff like Nirvana or Mudhoney. Personally I don't think there's really been many true alternative bands after 2001.