Alternative refers to a style of rock music popularized in the early 1990s that was viewed as an alternative to the rock music of the day.
Previously popular rock music:

New alternative:

Music now labeled alternative is viewed as somehow following in the tradition shown in the second photo. The alternative is mopey, odd, self-doubting whereas as the un-alternative is characterized by brash, macho swagger, reproductive fitness, and wailing solos. Musically, the alternative uses dissonance and grating-ness more often.
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The problem in defense is how far you can go without destroying from within what you are trying to defend from without.