Stairs, or a staircase 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." When Spock afterwards cautioned Kirk that he must not allow her to live, Kirk reminded him that Leonard McCoy was not present yet and therefore this was not the moment she was fated to die. Spock wasn't so sure. (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")