Stairs, or a staircase or stairwell was a set of steps that allowed one to move between levels of a structure.
In 1930, James T. Kirk instinctively caught Edith Keeler before she fell down the stairs and injured herself. She chided herself, saying she'd "been up and down those stairs a thousand times. I could have broken my neck." (TOS: "The City on the Edge of Forever")
Some stairs were reserved for emergency use only. In 1986, Mercy Hospital had such a flight of stairs, which the police officers chasing a fugitive Pavel Chekov and his rescuers used. (Star Trek IV: The Voyage Home)
While traveling with his son in a Bajoran lightship, Benjamin Sisko recalled how, homesick in his early days at Starfleet Academy, he would materialize in his living room at 6:30 every night and sit at the table as though he'd just come down the stairs. (DS9: "Explorers")