A "stair" refers to the individual step. A "stairway" and "staircase" mean the same thing and refer to a set of stairs between one floor and another. This usually refers to an open or semi-open set of stairs such as in a house. A "stairwell" usually refers to a set of stairs that are surrounded by walls, such as an emergency exit in an office building, hotel, or school.

These are nouns and they are singular not plural. Stair - a fixed series of steps. Stairway = Staircase - a stair and the supporting structure Stairwell - The walls surrounding the stairs (this is called the shaft) Also: Flight of stairs - staircase Landing - a flat platform between two staircases