A meta-human speedster and time traveler, under the superhero name XS, as well as the daughter of Barry Allen and Iris West-Allen from the future. She was stuck in the past for a while. Wackiness ensued.
When they reset time (due to their actions during the season), Nora was effectively killed and erased from existence. Her death led character motivations in the following season.
After the multiverse was combined and reborn (Crisis on Infinite Earths), Nora apparently was restored in time. Nora reveals that Barry and Iris would tell young Nora and Bart stories about previous timeline Nora and the impact she had on their family.
Although the restored Nora is alive and well, the death of Nora from season five was a significant event for the show so the death date remains on the character.