From The Art and Popular Culture Encyclopedia
In music, a single or record single is a type of release, typically a song recording of fewer tracks than an LP record or an album. This can be released for sale to the public in a variety of different formats. In most cases, the single is a song that is released separately from an album, but it usually appears on an album. Often, these are the most popular songs from albums that are released separately for promotional uses such as digital download or commercial radio airplay, and in other cases a recording released as a single does not appear on an album.