The term 'staircase piece' can be defined as a RISER, which is a five-letter word. A RISER is a vertical board or other material that forms the space between two steps in a staircase. It is typically used to provide structural support and stability to the staircase, as well as to create a uniform look. Additionally, risers are often used to hide any gaps between the steps, making the staircase look more aesthetically pleasing.