The title of the supreme religious leader of the Bajoran people, chosen from among the vedeks and held during Deep Space Nine by Opaka and then by Winn.
The kai is the head of the Bajoran faith and the most revered figure on Bajor. The office is filled by election from among the vedeks, and the kai's word carries weight in government as well as in the temples.
When Deep Space Nine begins in the episode Emissary, the kai is Opaka, who sends Benjamin Sisko to find the Celestial Temple and names him Emissary. She is lost to Bajor in Battle Lines, when she is left on a moon in the Gamma Quadrant.
The election that follows is the subject of The Collaborator. Vedek Bareil withdraws to protect Opaka's memory and Winn Adami becomes kai. She holds the office for the rest of the series, uses it to pursue political power in Shakaar, and at last turns from the Prophets to the Pah-wraiths.