Urban Dictionary to the rescue!
A word that would be useful if stupid poseurs didn't use it all the time.
That band alienated their fan base and changed their style according to what the record company wants, that means they're sellouts.
OMG! They sold 30,000 records? What sellouts!
OMG! They got on TV? Sellouts!
OMG! They're getting sponsored by some big company? Sellouts!
A vague but derogatory term blindly used by music "fans" to describe bands who make (or have managers who make) a new lucrative and/or artistically experimental decision (for example: accepting a product endorsement, a change in the band's expected composition style, switching to a larger record label, dressing with more class, speaking out against mass mp3 piracy, etc.) The word is sometimes also applied to bands who simply get more commercial success through no additional effort.
The anger comes from the false belief that fans "own" their favorite entertainers, that these entertainers are thus not real humans with free will, and the feeling of grief when some CD that the listener previously enjoyed alone is now enjoyed by millions of more people.
(Note that "sellout" ONLY applies to those who make a living in the fine arts. For example, a software engineer who accepts a job promotion and higher salary is never branded as a "sellout". Nor is a professional athlete who appears on a box of Wheaties.)
Actual quotes from an old friend:
(1993) "Damnit, Smashing Pumpkins is such an underrated band. They never get played on the radio!"
(1994) "Damnit, Smashing Pumpkins are always on Mtv and the radio now! They're such sellouts."
These definitions are amazing, why? Because you actually do get some idiots out there that think bands are selling out when they're not. They might change their sound because you can't have the same sound for a whole career, there has to be SOME type of progression. If you don't like the newer stuff or the music in a certain era produced by a band, then stick with the album or the era you liked then, don't bring down the band as a whole because of it!
The other thing besides the sound of the music that people seem to say is a sign of selling out is that they're selling hundreds of thousands of albums... so as an artist you DON'T want your music to get noticed? It's meant to be held hostage, not allowed to see the light of day? That's stupid if I ever heard it! You'd want everyone and anyone to hear your music, and believe it or not, that's through the radio, the tv, advertisement, playing big venues!
All these fools that label a band sellouts, be a fan and support them no matter what, if you don't like them any more, move on to a "better" band, don't ruin it for real fans that stick by them through thick and thin!