In a coven, she is usually the female leader of the group who co-leads with the High Priest. A High Priestess has obtained a level of proficiency through years of study in her chosen tradition, and has been initiated and passed several "rites of passage." In Wiccan, neither the High Priest, or the High Priestess are dominant, they are joint leaders and equals.
Technically speaking, a female Witch who has received the 3rd. degree initiation. More usually, the female leader of a coven or sometimes just of that particular ritual. Abbreviated as HPS.