Selling out refers to the compromising of one's integrity, morality and principles in exchange for money, 'success' or other personal gain. It is commonly associated with attempts to increase mass appeal or acceptability to mainstream society. A person who does this, as opposed to following the original path s/he laid (or claimed to lay) out for him/herself, is labeled a sellout and regarded with disgust and immediate loss of respect. Selling out is seen as gaining success at the cost of credibility.

In various political movements, such as the communist and anarchist movements, a "sellout" is someone or some group who pretends to adhere to a sincerely pro-working class ideology, only to follow these claims up with actions that directly contradict them and that may even go so far as to actually or implicitly support capitalism. It could also apply to a revolutionary group who claims to fight for the people of a country but acts far different upon coming to power, mostly because the covert goal of the revolution was not to benefit the people of the nation, but for the national government to be overthrown so the revolutionary leaders could have all the perks and prestige of being in power.

An unrelated use of this phrase in regard to theater shows, musicals, concerts and other performances, a sell out show is one for which all available tickets have been sold, and is generally considered a measure of success and popularity. Similarly, a merchant can be "sold out" of a particular product of any kind, meaning that at the given time, every unit the merchant had in stock of that product has been purchased.