The Single would best be defined as a one or two-song release of tracks from an artist, but life is never that simple!
Staring with the 50s, the single was a 45rpm 2-song release on the 7″ vinyl format. In later years, 7″ records that played at 33 1/3 RPM and contained more tracks were called EPs.
In the 70s came the 12″ (or Maxi) single, predominantly used for Reggae and disco tracks. This became the standard format for much dance music. In the UK, controversy first arose with the release of Steve Silk Hurley’s “Jack Your Body” in the Late 80s. Clocking in at 24 minutes this release upset the chart compilers who felt it should not be classified as a single.
A game of cat-and-mouse has ensued between record labels and chart compilers which has seen a single nailed down to a maximum song length of 25 minutes (hence the Orb’s “Blue Room” being 24 minutes and 59 seconds) and a maximum of 4 different songs. The Advent of CD has made the definition of a single a little harder. While 4 different songs is the limit; no end of remixes can be fitted on a single. Many modern dance CD singles have up to 7 mixes and are longer in total than many albums!
Conceptually, at least, the single is generally understood to be an individual song that is released and promoted in the marketplace, usually as a selection from a complete recording of some form.