The Apparition by Gustave Moreau (c.1874-76)

Apparitions

Definition: Something that unexpectedly appears or becomes visible, apparently not by natural means.

Etymology: Middle English apparicioun, from Anglo-French aparicion, from Late Latin apparition-, apparitio appearance, from Latin apparēre.1

"Apparition" is a blanket term used to describe many different types of unexplained visual phenomena. Apparitions can be roughly divided into the following categories:

Ghosts (phantoms, phantasms): The disembodied manifestations of the spirits or souls of deceased humans or animals that survive after death.2,3
Hauntings: Occurrences of paranormal activity associated with a particular location or structure and usually attributed to the activities of an anomalous entity.4
Orbs: A type of anomaly that appears in photographs, in which mysterious objects appear floating in the air.
Doppelgänger: A wraith or apparition of a living person; a "spirit double" or exact replica of a person5.
Bilocation: The phenomenon of a person appearing in more than one place simultaneously.
Religious visions: The appearance of a deity (god) or holy person such as Christ, saints, prophets or the Virgin Mary (known as a Marian apparition).

Resources:

1. http://www.merriam-webster.com/dictionary/apparition
2. Supernatural Glossary. 2008. Psychic Experiences. 27 March 2008. <http://www.psychic-experiences.com/glossary.php>
3. ParaNormal and Supernatural Definitions. [unknown]. ParaSkeptics. 27 March 2008. <http://www.wrexhamparaskeptics.4t.com/definitions.htm>
4. Glossary of Psi (Parapsychological) Terms (E-K). 2008. Parapsychological Association. 24 March 2008.<http://www.parapsych.org/glossary_e_k.html#g>
5. "doppelgänger." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 10 May 2008 .