Stairs, also called a staircase, stairwell or steps, were 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)
Stairwells were part of the underground catacombs beneath the monastery of P'Jem. (ENT: "The Andorian Incident")
A stairwell connected Garos' curio shop with the underground mining operation center in his basement on the Akaali homeworld in 2151. (ENT: "Civilization")
Beside several ladders, the launch bay aboard the NX-class Enterprise NX-01 featured stairs to get down from the second level to the shuttlepods. (ENT: "The Catwalk")
While Spock's katra was stuck inside Leonard McCoy, he told James T. Kirk to "climb the steps of Mount Seleya," so as to remove the katra from his mind. (Star Trek III: The Search for Spock)
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")
At one point, a central staircase was considered as one of the features of the Galaxy-class.