Riser

Riser –  The vertical component of a step. The treads rests on the risers. Each step is composed of the tread and riser.

A staircase or a section of stairs will have 1 more riser than tread since there is one at the bottom and another at the top before reaching a landing or 2nd floor hall. (the landing nose or hall/floor nose rests on the top riser.) 

This is important when calculating footage for a stair runner - we refer to stair counts as number of treads/number of risers. Example: a single run on a straight staircase with 12 treads has 13 risers. When there is a landing within the staircase, the thread/riser counts should be broken down by each flight or run. 

The total number of risers is always greater than the total number of treads.