Classic song - a song considered as "classic" by most hip-hop heads
seriously it's hard to define but I'll try to say what's a "classic song" to me. I think it's a song that distinguishes itself from other songs by being more controversial or just has great beat and lyrics and is admitted "classic" by public.
classic album - album having more than xx percent of good songs or few "classic songs" on it.
Also if somebody calls a new heard track a "classic" it means it has somehow important meaning to that person, is so funny or so deep or has all above combined.
yeah I know my definisions suck but that's how I see it.