A priestess is a female religious figure. In ancient Greece, a priestess often dressed in the style of a goddess, wearing white robes and carrying a long staff.
The word priestess is a feminine version of priest, which stems from the Old English prēost and its Greek root, presbyteros, "an elder." While hundreds of years ago a priestess was simply a female priest, today's Christians use priest whether they're talking about a man or a woman. The word priestess is used for ancient religions and occasionally pagan or Wiccan religious leaders.