In music, accompaniment is the art of playing along with an instrumental or vocal soloist or ensemble, often known as the lead, in a supporting manner. The accompaniment can be performed by a single performer — a pianist, organist, or guitarist — or it can be played by an entire ensemble, such as a symphony orchestra or string quartet (in the classical genre), a backing band or rhythm section (in popular music), or even a big band or organ trio (in jazz). It may be considered the background to the foreground melody.
The term accompaniment also describes the composed music, arrangement, or improvised performance that is played to back up the soloist. In most Classical styles, the accompaniment part is written by the composer and provided to the performers in the form of sheet music. In jazz and popular music, the backing band or rhythm section may improvise the accompaniment based on standard forms, as in the case of a small blues band or a jazz band playing a 12-bar blues progression, or the band may play from a written arrangement in a jazz big band or in a musical theater show.
The accompaniment part usually provides the harmonic background and the rhythmic structure for the piece of music or song. The harmonic background is usually provided by one or more instruments that play a chord progression. Instruments commonly used to play chords, also called harmonic accompaniment, include the acoustic or electric guitar, piano, organ and electronic keyboards. An accompaniment can also be provided by instruments that normally play the melody, such as the violin.
The accompaniment often includes a bass instrument (bass guitar, upright bass, etc.) that plays the bass notes of the harmonic progression. The rhythmic structure of the piece or song is typically provided by drums or percussion in most types of popular music, jazz, and blues. In Classical music styles, many types of pieces do not include percussion instruments, such as string quartets and organ trios.
In most tonal music the melody and accompaniment are written from and share the same group of pitches, while in much atonal music the melody and accompaniment are chosen from entirely separate groups of pitches, often from different hexachords. Basso continuo is a form of notation used especially in Baroque music accompaniment parts.
Accompanist is one who plays an accompaniment. A number of classical pianists have found success as accompanists rather than soloists; arguably the best known example is Gerald Moore, well known as a Lieder accompanist. In some American schools, the title collaborative pianist (or collaborative artist) is replacing the title accompanist.
The term accompanist also refers to a musician (typically a pianist) who plays for singers, dancers, and other performers at an audition or rehearsal—but doesn't necessarily participate in the ensemble that plays for the final performance.
Accompaniment figure 
An accompaniment figure is a musical gesture used repeatedly in an accompaniment, such as:
- Alberti bass and other arpeggio figures
- Ostinati figures (repeated lines) or, in popular music, riffs
Dialogue accompaniment 
Dialogue accompaniment is a form of call and response in which the lead and accompaniment alternate, the accompaniment playing during the rests of the lead and providing a drone or silence during the main melody or vocal.
Other Definition 
Accompaniment (Food) is something that you eat or drink with something else: 'A dry champagne makes the ideal accompaniment for/to this dish.' It is also something added to complete or embellish or make perfect. Consider this: if your menu tonight consists of grilled pork and savory apples, then the pork is the main item and the apples are the accompaniment, meaning they complement the main dish.
See also 
The dictionary definition of accompaniment at Wiktionary