apparition

Short definition from Wiktionary

Noun

1. 1
   An act of becoming visible; appearance; visibility.
2. 2
   The thing appearing; a visible object; a form.
3. 3
   An unexpected, wonderful, or preternatural appearance; especially, one by a ghost, phantom, or suchlike; the entity itself.
4. 4
   The first appearance of a star or other luminary after having been invisible or obscured; opposed to occultation.
5. 5
   A period of consecutive days or nights when a particular celestial body may be observed, beginning with the heliacal rising of the body and ending with its heliacal setting.