When referring to the person playing piano with you, call them your “pianist.” (Emphasis on the “a” so as to not be accidentally confused with a member of the male anatomy.)
Please do not call your pianist an “accompanist.” We do more than “accompany.” On average, we have to work as hard on your pieces as you do. More importantly, we have to know your part as well as you do – even if you don’t know ours. We are following your every move and calculating what you will do next. We follow your breaths. If we sense you will not make the phrase, we will push the tempo ever so slightly to help you out. If you make a mistake, we cover it. If you miss an entrance, we cover it. If you jump ahead or back, we cover it. If you do something you’ve never done in rehearsal, we accommodate. We balance our sound to the hall we’re in so as to showcase you. We voice our parts to your advantage. “Accompanist” is, honestly, disrespectful and dismissive. We are pianists. Collaborative pianists, when we play with you. But we are always pianists.
UNDER NO CIRCUMSTANCES will you ever call your (or anyone else’s) pianist “accompanyist” or “accompanimist.” They are not real words. They are childish. We will embarass you for it, and we reserve the right to walk out on the spot.