Stairs are constructed series of steps which are staggered so that a person can use them to walk up or down between levels of varying height. These can exist inside or outside a structure, and can be assembled or carved in situ. Statistics on stairs may refer to stairwells, stairways, individual flights of stairs, or the steps themselves. Other constructions for ascending and descending, such as mounted ladders, are not considered stairs.

The steps can be assembled from separate elements or carved in place. Their walking function and stepped form distinguish them from a ladder or a smooth ramp.

A flight is one sequence of steps, while a stairway can contain several flights. A stairwell is the space housing stairs, so counts of these different elements need not agree.