A mortal was any being that was vulnerable to dying, as opposed to immortal beings. The term was often used by immortals like the Q Continuum philosopher Quinn, or the Greek god Apollo, to refer to beings not sharing their immortality.
Apollo noted that his kind, which was immortal, had mated with mortals. His Human mother, Leto, was a mortal. (TOS: "Who Mourns for Adonais?")
In 2366, Q was briefly turned into a mortal by the Q Continuum. (TNG: "Deja Q")
After Data's head was found on an dig in San Francisco in 2368, he mused about his mortality, which he found comforting. Ultimately, he declared; "One might also conclude that it brings me one step closer to being Human. I am mortal." (TNG: "Time's Arrow")
After a Mintakan population started seeing Starfleet personnel as deities, Jean-Luc Picard intervened, in part by arguing that he was as mortal as they were. (TNG: "Who Watches The Watchers")
After Seven of Nine explained to Tuvok that the Borg were immortal in a way because after death their memories continued to exist, Tuvok argued that since she was no longer connected to the collective, she had become mortal. (VOY: "Mortal Coil")