The High Priestess of the Temple of Mount Seleya was a title held by the senior Vulcan priestess who lived at Mount Seleya.
During ancient Vulcan history before the time of the Sundering, Surak and his comrades went on a pilgrimage to Mount Seleya in order to gain the help of its Adepts and the Adepts of Gol for helping him in his plan to send a number of their kind away from the Mother World in order to preserve their ways in case they destroyed themselves in war. Upon reaching the temple, the High Priestess was present and was attended by the Unbonded; young females who were not married and served as her attendants whilst they were training in the healing arts. When Surak petitioned the High Priestess on whether she would leave, she refused and stated that she was one with Vulcan and would rather die on it then leave. Furthermore, she stated that they would not take part in the conflict if a devastating war erupted on Vulcan. Before Surak left, she handed him what she considered a blasphemous Coronet and asked him to remove it from her presence. (TOS - Vulcan's Soul novel: Exodus)
The most powerful High Priestess was known to have been T'Pau who was succeeded upon her death by the First Adept, T'Lar of Gol. (Last Unicorn RPG module: The Way of Kolinahr: The Vulcans)