Back in the 80's the term "indie" often refered to bands that had the "independent" punk mentality but still embraced the aesthetics of pop. In fact, indie used be more implicit of indie-pop and twee-type music (probably the most famous of which are Belle & Sebastian and Television Personalities). It wasn't until bands that fit into the whole indie-pop sort of category began adding more rock influence that the term began to expand and get more ambiguous.
Most indie rock bands today are heavily influenced by new wave and dancy rock. In fact, most bands that are all the rage in indie music are little more than derivatives of older bands that people have forgotten about. Interpol is basically an updated Joy Division; Franz Ferdinand/Bloc Party/The Futureheads/et cetera all sound like Gang of Four with some Television mixed in.
The term is pretty loose now. Basically "indie" music is defined by what indie music fans listen to. And indie music fans are basically anyone who likes strong melodies and pop sensibilities too much to immerse themselves in the punk/hardcore scene and like creativity and originality too much to become immersed entirely in radio pop. That said, because indie music is defined by what indie-kids listen to, it is quite possible for radio pop artists and crusty punk bands alike to gain indie appeal in certain situations. I like to think of indie as a moderate view in music; indie-kids are the ones that like to dress differently, but not cover themselves in studs and safety pins; musically, the genre usually borrows the good elements from both extremes (and often into other genres as well, as hip-hop is pretty popular in indie circles too).
There is a lot of subjectivity involved and not really any concrete definitions about what is and is not indie.
...zip and email maybe?
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