Stairs
Stairway
noun
The floor that makes up the steps are the stairs. Don't get this confused with the similar word 段階, which is the stairs making up the floors to a certain stage.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
充は、私が階段の掃除を手伝った事に礼を述べた。
Mitsuru thanked me for helping him clean the stairs.