The term 'Stair UNIT' is a combination of two words, 'STEP' and 'UNIT'. STEP is a four-letter word which refers to the individual steps that make up a staircase. A UNIT is a single component of a larger structure or system. Therefore, a Stair UNIT is a single component of a staircase, such as a tread, riser, baluster, or handrail. It is important to note that a Stair UNIT is not the entire staircase, but rather a single part of it.