An instrumentalist is a person who plays a musical instrument — piano, violin, guitar, drums — rather than singing. In a concert you might hear “the singer and the instrumentalists.” It is a neutral, slightly formal word. Everyday talk often just says player, or names the instrument (a guitarist, a pianist). Though actually, in philosophy, instrumentalist also names a kind of thinker — but that is a different, much rarer use, and this entry is the musician. The word is built on instrumental, “of instruments.” It is not the person who was “instrumental” in a plan (very helpful in making it happen). That helpful sense of instrumental is a different road, and it does not make you an instrumentalist.