What does the name indie mean?

A song is indie if it is on a independent label. In actuality this is more complicated than it seems because some indie bands join major labels but are still respected by the indie culture. For example, the band Sonic Youth have been on a major label since the 90's but many people still respect them and call them indie. Indie may also refer to a specific style of music. In that respect indie is an umbrella term meaning that it encompases many sub genres and styles. Usually, trends birth these sub genres and styles. So when someone calls a song indie they may refer to a certain trend in music as well as whether the artist is on a major label or not.