Stairs, also called 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)
Severals stairwells connected the ground level and the second level of the Promenade aboard Deep Space 9. One stairwell was located next to the Replimat and another one was inside Quark's. (DS9: "Meridian" et al.)
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")