The accompaniment supports the melody. It can be called the background music. The accompaniment enhances that melody as well.
Instruments or vocals---or both together---perform an accompaniment. Solo and ensemble singers perform vocal accompaniments.
Some people believe that a capella songs have no accompaniment. However, they only lack instrumental accompaniment. Many a capella songs have an accompaniment performed with vocal music.
Instrumental music isn't just accompaniment music. The song will consist of a single instrument or family of instruments playing the melody.
According to the Encyclopedia Britannica, early accompaniments were just copies of the melody, changed just enough to be distinct. Other accompaniments were one long note or set of notes played on a string instrument like a violin or on a woodwind instrument like a clarinet.