Devi is a Sanskrit term meaning the female aspect of divinity, close to the English word Goddess. She is the Divine Mother: energy, or Shakti, without which the male aspect—consciousness—is powerless. The old saying is that Shiva is shava (a corpse) without Shakti.
Within the International Nath Order, Devi is also the suffix given to an initiated woman. Initiates of the Order are known as Naths or Devis. Shri Gurudev Mahendranath wrote that women are usually called Devi and rarely Nathess. The cryptonym is given at initiation. It marks the initiate. It does not claim that she is the Goddess, and it is not another word for Shakti. Shakti is the current. Devi, in this second use, is the woman who has received the transmission.