noun

• (astronomy) An astronomical event that occurs when one celestial object is hidden by another celestial object that passes between it and the observer when the nearer object appears larger and completely hides the more distant object.
  "[T]he diſtance of any place within this kingdom from it, will not much vary the manner of their Appearance in any of the Phænomena, except the Eclipſe of the Sun: for, in the Occultations, the Stars will appeare to paſs nearly under the ſame Angles and Spots of the Moon; […]"
• (Shia Islam) The disappearance of the Twelfth Imam, or Mahdi, who is believed alive and present in this world, but hidden until his reappearance at the end of time.
  "We are therefore, constantly hoping for the joy and happiness, personified in Imām al-Mahdi, the Imām who is alive at this very time, and who is himself awaiting the command of Allāh to reappear, so that he may strengthen the weak, and judge the oppressors. He will reappear "without having done Bay'ah to any oppressor". He went into occultation because he did not want [to] submit to any unjust ruler."