"Alternative music" is a broad term used, since the late 1960s, to describe popular music seen as less commercial and mainstream, and more authentic, original, or eclectic. It embraces an aesthetic that rejects the commercial music industry, and places an emphasis as art or expression rather than as a product for sale or economic profit. Having said this, bands such as Radiohead, Nirvana, and U2, which are often categorized as alternative, have also been among the most commercially successful.
Alternative music includes alternative forms of all the genres of popular music, including rock, folk, and country. It can also refer to artists of several musical styles as it's not tied to only one style. Alternative music tends to be concerned with "the message" rather than conforming to what the industry deems as popular at the time.
The use of "alternative" emerged in response to the co-option of rock music by the music industry in the late 1960s and through the 1970s. The term was originally used in the late 1960s to refer to UK and US underground or counter-culture performers. The broad genre was closely associated with independent record labels, and was accordingly sometimes referred to as indie music.
Punk in the late 1970s was alternative, and the subsequent styles identified as alternative (for example, American hardcore and 1980s indie rock) built on punk. The alternative label has also been applied to the grunge bands of the late 1980s and 1990s, and remnants of the underground/counter-culture. Alternative rock has itself been subject to mainstreaming, however, with the moves of leading performers, such as U2, Nirvana, and Radiohead from independent labels to major companies.