In music, a single is a type of release of a song recording of fewer tracks than an album (LP), typically one or two tracks. A single can be released for sale to the public in a variety of physical or digital formats. Singles may be standalone tracks or connected to an artist's album, and in the latter case would often have at least one single release before the album itself, called lead singles.

The single was defined in the mid-20th century with the 45 (named after its speed in revolutions per minute), a type of 7-inch sized vinyl record containing an A-side and a B-side, i.e. one song on each side. The single format was highly influential in pop music and the early days of rock and roll, and it was the format used for jukeboxes and preferred by younger populations in the 1950s and 1960s.

Singles in digital form became very popular in the 2000s. Distinctions for what makes a single have become more tenuous since the biggest digital music distributor, the iTunes Store, only accepts as singles releases with three tracks or fewer that are less than ten minutes each (with longer releases being classified as EPs or albums). However, releases which do not fit these criteria have been promoted as singles by artists and labels elsewhere, such as on Spotify and the Bandcamp storefront.

Nowadays physically-released music is mainly bought in the form of full-length albums instead of singles. The most common physical formats of singles had been the 7" (45) vinyl records and the CD single, but singles have also been released on other formats such as 12" vinyl records, 10" shellac records, cassette single, and mini CD.

The origins of the single are in the late 19th century, when music was distributed on phonograph cylinders that held two to four minutes' worth of audio. They were superseded by disc phonograph records, which initially also had a short duration of playing time per side. In the first two to three decades of the 20th century, almost all commercial music releases were, in effect, singles (the exceptions were usually for classical music pieces, where multiple physical storage media items were bundled together and sold as an album). Phonograph records were manufactured with a range of playback speeds (from 16 to 78 rpm) and in several sizes (including 12 inches or 30 centimetres). By about 1910, however, the 10-inch (25 cm), 78-rpm shellac disc had become the most commonly used format.

The inherent technical limitations of the gramophone disc defined the standard format for commercial recordings in the early 20th century. The relatively crude disc-cutting techniques of the time and the thickness of the needles used on record players limited the number of grooves per inch that could be inscribed on the disc surface and a high rotation speed was necessary to achieve acceptable recording and playback fidelity. 78 rpm was chosen as the standard because of the introduction of the electrically powered synchronous turntable motor in 1925, which ran at 3,600 rpm with a 46:1 gear ratio, resulting in a rotation speed of 78.3 rpm.

With these factors applied to the 10-inch format, songwriters and performers increasingly tailored their output to fit the new medium. The three-minute single remained the standard into the 1960s, when the availability of microgroove recording and improved mastering techniques enabled recording artists to increase the duration of their recorded songs. The breakthrough came in the U.K. with The Animals "The House of The Rising Sun" of 4 minutes 29 seconds (cut down to 2 minutes 59 seconds for the U.S.A. market) in 1964, followed by Bob Dylan's "Like a Rolling Stone" in 1965: although Columbia Records tried to make the record more radio-friendly by cutting the performance into halves and separating them between the two sides of the disc, both Dylan and his fans demanded that the full six-minute take be placed on one side and that radio stations play the song in its entirety. The next milestone, which cemented pop radio's tolerance for longer songs, was the Beatles "Hey Jude", which was released August 1968 and clocked in at over seven minutes, pushing the limits of the 45 rpm single.

Singles have been issued in various formats, including 7-inch (18 cm), 10-inch and 12-inch discs, usually playing at 45 rpm; 10-inch shellac discs, playing at 78 rpm; maxi singles; 7-inch plastic flexi discs; cassettes; 8 or 12 cm (3.1 or 4.7 in) CD singles, and less commonly on Digital Compact Cassette and many non-standard sizes of vinyl disc (5 in or 13 cm, 8 in or 20 cm, etc.). Singles were also released on DVD, CD Video, and Laserdisc video formats, which would additionally contain music videos to be watched on a display such as a television set.