A staircase unit is a set of stairs that are connected to form a single structure. It is typically used to provide access between two levels of a building, such as between the ground floor and the first floor. The term 'staircase unit' is often used interchangeably with 'stairway, ' although the latter term is more commonly used to refer to a single flight of stairs. The crossword solution for the term 'staircase unit' is 'STEP, ' which refers to the individual steps that make up the staircase unit.