
Hidden track

In the field of recorded music, a hidden track (sometimes called a ghost track or a secret track or an unlisted track) is a song or a piece of audio that has been placed on a CD, audio cassette, LP record, or other recorded medium, in such a way as to avoid detection by the casual listener. In some cases, the piece of music may simply have been left off the track listing, while in other cases more elaborate methods are used. In rare cases a 'hidden track' is actually the result of an error that occurred during the mastering stage production of the recorded media. 
It is occasionally unclear whether a piece of music is 'hidden'. For example, "Her Majesty", which is preceded by fourteen seconds of silence, was originally unlisted on The Beatles' "Abbey Road", but is listed on current versions of the album. That song and others push the definition of the term, causing a lack of consensus on what is considered a hidden track. Alternatively, such things are instead labeled as vague audio experiments, errors, or simply an integral part of an adjacent song on the record.
Techniques.
A vinyl record may be double-grooved, with the second groove containing the hidden tracks. Examples of double-grooving include Monty Python's 'three-sided' "Matching Tie and Handkerchief", Tool's "Opiate" EP, and Mr. Bungle's "Disco Volante".
Grooves cannot be read from digital media such as compact discs, so alternate methods were conceived. With a similar aim of concealment, unlisted tracks are sometimes given their own separate index point on digital media. Songs can be placing in the pregap of the first track of certain CD formats, so that the CD must first be cued to the track, and then manually back-scanned. These are often referred to as Track 0 or Hidden Track One Audio (HTOA). A CD player will not play these tracks without manual intervention, and some models (including many computers operating systems) are unable to read such content. On Super Furry Animals' third studio album, "The Citizens Band" is found in the pre-gap approximately 5 minutes before the beginning of track 1. A glossary of terms used in the song's lyrics are printed on the interior of the cardboard outer sleeve of the CD. This essentially renders them inaccessible without taking the sleeve apart, hiding the glossary in a parallel way to the song itself. "See List of albums with tracks hidden in the pregap."
A less concealed method is to place the song at the end of another track, typically the last track on the album, following a period of silence. For example, Nirvana's song "Endless, Nameless" was included as a hidden track in this way on their 1991 CD "Nevermind", after 10 minutes of complete silence within the track listed as the final song. Although it was not the first hidden song to use this technique, it gained significant attention. Similarly, short tracks of silence can be layered before the hidden track plays. On Lazlo Bane's debut album, 11 Transistor, the eleventh song is followed by 57 silent tracks 4 seconds each, with "Prada Wallet" (sometimes referred to as "The Birthday Song") being the 69th track on the album. The total length of silence between the two songs is 3:48.
It is possible for a track to be playable only through a computer, such as the '15th' track on Marilyn Manson's "Mechanical Animals" album, which can only be accessed through an Enhanced CD executable.
There are yet-deeper ways a track can be hidden. A "ghost" track can be subtly mixed to play concurrently with other, dominant audio, or heavily distorted in a way which must be undone to be played. For example, on a DVD included with the deluxe and 'ultra-deluxe' editions of Nine Inch Nails' "Ghosts I–IV", two hidden bonus tracks ("37 Ghosts" and "38 Ghosts") are included as digital multitrack files, from which the songs may be reconstructed.
Notable hidden tracks.
Sometimes hidden tracks have become well known and received radio airplay, and occasionally climbed the charts.