A variant or archaic form of accompanist; a musician who provides instrumental support to a soloist.
From accompany (not accompaniment) + -ist suffix, representing an older way of forming this word. It's essentially a different path to create the same profession.
This is a beautiful example of how English lets us build the same word multiple ways—accompanyist (from the verb) and accompanist (from the noun) both work, but one became standard and the other is mostly historical.
Complete word intelligence in one call. Free tier — 50 lookups/day.