A rather obsolete term people use to describe bands that have become more mainstream/commercial, usually after signing to a major label. Of course, just because a band has become more mainstream doesn't mean they've 'sold out'- it just means they have more money to spend on professional recordings, greater exposure, etcetera. And as long as a band does not prioritise money over their artistic vision then they should not be accused of wrongdoing. And the fact is, the majority of people who accuse bands of selling out are just irate at the fact that their music is no longer exclusive to them and the niche audience that they can brag about being a part of. So long as money exists people will always have a reason to accuse a band of being in cahoots with men in black suits. Besides, a lot of landmark albums in specific genres have been released when a band had signed to a major label. A band can't help it if their music appeals to the mainstream, so give them a break. And the only artists who should be accused of selling out are those manufactured by heartless record execs with a fetish for money, not those who set out to be a band and sign to a major to gain greater exposure and the recognition they deserve. At the end of the day, everyone's a sellout anyway: we all support the establishment whether we intend to or not, so don't forget that next time you target another band with the 'selling out' ethos.
Dude 1: God, why are you listening to Green Day? they've totally sold out since signing to Warner Bros. You should listen to the Sex Pistols, at least they're not some commercial wannabe rock band.
Dude 2: You do realise that the Sex Pistols were signed to EMI, Virgin and even Warner Bros. right?
Dude 1: (remains silent)
Dude 2: Exactly... so your 'selling out' arguments a bit naive isn't it?