The Collaborative International Dictionary of English v.0.48:

Accompanist \Ac*com"pa*nist\, n.
   The performer in music who takes the accompanying part.
   --Busby.
   [1913 Webster]

WordNet (r) 3.0 (2006):

accompanist
    n 1: a person who provides musical accompaniment (usually on a
         piano) [syn: accompanist, accompanyist]