A misnomer for a particular style of music. It formerly was used to correctly classify music that was "underground" and not supported by major music labels. Popular thought in recent years has been that it is itself a style of music, usually 80's throwback, alternitive-style music.
Any band in any style of music can be indie by virtue of not being signed.The style is not important; it is independence from major labels that makes a band independent, or "indie". This idea has been lost due to the popularity of the term indie as it is used today.
The reason it became so popular is because to like indie music, in its correct sense, was respectable because you're bypassing the corporate music machine and finding music that you like on your own. As more and more hipster-type kids wanted to appear as deep as possible, they would answer the question "What kind of music do you like?" with "Indie." As time went on, this answer became so popular that the music that many of them listened to started to be referred to as indie. After a while of this, bands that are on major labels started to be absorbed into this pseudo-genre, rendering it an oxymoron.
Since the loosely associated music that is referred to as indie cannot properly be grouped and classified, a more appropriate label for what is erroneously referred to as indie would be "scene." The scene
kids are often more concerned with being opinionated and elitist about their music choice than they are about the music itself. Scene fashion trends include tight pants, square-rimmed glasses, tight vintage-looking t-shirts, newsboy caps, military-style caps, and tattoos of stars, guns or bullets, and especially birds. Making the "scene" (i.e. a concert),being popular, and being narcissistic are the main motivations of scene kids. They often do their best to display that they possess these qualities on social networking sites like MySpace.