An accompanist is the musician who plays along while another person sings or plays the main tune. In real life that person is very often at the piano, though it can be another instrument. The job is support, not the spotlight — though actually a good accompanist is not ‘just background’. They listen, they wait, they catch the singer if the speed slips. The word belongs to concert programmes and music-school talk. In ordinary chat, people often just say piano player or ‘the person on piano’.