She practiced violin since childhood and has now become a professional violinist.

violin 是名词，指"小提琴"，一种四弦的弓弦乐器。 violinist 是由 violin 加后缀 -ist 构成的名词，-ist 表示"从事某种职业的人"，所以 violinist 指"小提琴演奏者"或"小提琴家"。