An apparition is an act or instance of appearing.

Supernatural

An apparition is an appearance to a human of a ghost, phantom, or other supernatural being such as God or Satan, or an appearance of a living person not physically present by supernatural means. In loose speech, most people take the word apparition to mean simply a ghost.

In Roman Catholicism, a number of apparitions of the Virgin Mary are celebrated.

Astronomical

Astronomers call certain sightings of celestial bodies apparitions.