The term 'stair segment' can be broken down into two parts: the RISER and the tread. The RISER is the vertical part of the stair, while the tread is the horizontal part. The crossword solution for the term 'stair segment' is 'RISER, ' which is a five-letter word. The RISER is the part of the stair that provides support and stability to the tread, and it is typically made of wood or metal. It is important to note that the RISER is not the same as the tread, as the tread is the part of the stair that is stepped on.