Ghost notes are musical notes occurring in a rhythmic figure which are purposely deemphasized, often to the point of near silence. In popular music drumming these notes are played, "very softly between the 'main' notes," most often on the snare drum in a drum kit:
A rhythmic figure may be punctuated by certain notes which are accented (emphasized), in which case we would say that the unaccented notes in the figure are played with a 'normal' degree of emphasis. This is the case, for instance, in a clave pattern whose notes are spaced evenly across time but with certain notes in the pattern receiving a degree of emphasis which makes them stand out from the others in terms of volume. Therefore, in a clave pattern, we have accented as well as unaccented notes. (Some might argue that a clave pattern is really only made up of the accented notes, with any unaccented notes being placeholders and not part of the pattern. This is open to debate, and depends largely on instrumentational factors.)
Ghost notes, however, are not simply the unaccented notes in a pattern. The unaccented notes in such a pattern as a clave are considered to represent the mean level of emphasis—they are neither absolutely emphasized nor unemphasized. If one further deemphasizes one of these unaccented notes to the same or a similar extent to which the accented notes in the pattern are emphasized, then one has 'ghosted' that note. In a case in which a ghost note is deemphasized to the point of silence, that note then represents a rhythmic placeholder in much the same way as does a rest. This can be a very fine distinction, and the ability of an instrumentalist to differentiate between what is a ghost note and what is a rest is governed largely by the acoustic nature of the instrument.
Wind instruments, including the human voice, and guitars are examples of instruments generally capable of ghosting notes without making them synonymous with rests, while a pianist or percussionist would have more difficulty in creating this distinction because of the percussive nature of the instruments, which hampers the resolution of the volume gradient as one approaches silence. However, in such a case as that the ghost notes were clearly audible, while being far less prominent than the unaccented notes which represent the mean degree of emphasis within the example, then a percussionist could be said to create what we might define as ghost notes.
A frequent misconception is that grace notes and ghost notes are synonymous. A grace note is by definition decidedly shorter in length than the principal note which it 'graces', but in many examples the grace note receives a greater degree of accentuation (emphasis) than the principal itself, even though it is a much shorter note than the principal. In other words, while a grace note could be ghosted, the ghosting of notes is a function of volume rather than of duration.
The term ghost note, then, can have various meanings. The term anti-accent is more specific. Moreover, there exists a set of anti-accent marks to show gradation more specifically. Percussion music in particular makes use of anti-accent marks, as follows:
A guitarist wishing to ghost a note can decrease the pressure the fretting hand is exerting upon the strings without removing the hand from the fretboard (which would result in the sounding of the open pitches of those strings). This is sometimes called a 'scratch', and is considered a ghost note unless all the unaccented notes in the pattern were 'scratched' (in which case the scratches are unaccented notes).
On the double bass and electric bass, as with the guitar, ghost notes can be performed by muting the strings, either with the fretting hand or the plucking/picking hand, which creates notes of indeterminate pitch that have a percussive quality. On the electric bass, ghost notes are widely used in the slap bass style, as a way of creating a percussive, drum-like sound in funk and Latin music. On the double bass, percussive ghost notes are sometimes performed by slapping the strings against the fingerboard, which creates a percussive, "clacky" sound. With the double bass, slap-style ghost notes are used in rockabilly, bluegrass, and traditional blues and swing jazz.