Stairs are the steps you climb to go from one floor to another. A flight of them, with a rail at the side.
The word is almost always plural, and the singular stair is used mainly in poetry or in a phrase like the bottom stair. The structure as a whole is a staircase or a stairway, and the flat bit at the top is the landing. English says upstairs and downstairs as single words, and below stairs once meant the servants' part of a house. A spiral staircase turns as it climbs.
1nounstairsA set of steps leading from one floor to another.
“She ran down the stairs two at a time.”
Also accepted in games: staircase