..that makes 'alternative' music 'alternattive'? Is there such a thing?

I was chatting to somene the other day and she asked what sort of music I was into. I didn't want to come across as an indie snob so I avoided the 'oh, you probably haven't heard of them' route. So I just went with 'alternative'. Then she says to me: 'so what's that exactly?' Crikey - did I have an answer for her? So I rambled off some bands but....

the antidote to mainstream or something more?