In traditional belief and fiction, a ghost is the soul or spirit of a deceased person or animal that can appear, in visible form or other manifestation, to the living. Descriptions of the apparition of ghosts vary widely from an invisible presence to translucent or barely visible wispy shapes, to realistic, life-like visions. The deliberate attempt to contact the spirit of a deceased person is known as necromancy, or in spiritism as a séance.
The belief in manifestations of the spirits of the dead is widespread, dating back to animism or ancestor worship in pre-literate cultures. Certain religious practices—funeral rites, exorcisms, and some practices of spiritualism and ritual magic—are specifically designed to appease the spirits of the dead. Ghosts are generally described as solitary essences that haunt particular locations, objects, or people they were associated with in life, though stories of the phantom armies, ghost trains, phantom ships, and even ghost animals have also been recounted.
A widespread belief concerning ghosts is that they are composed of a misty, airy, or subtle material. Anthropologists link this idea to early beliefs that ghosts were the person within the person (the person's spirit), most noticeable in ancient cultures as a person's breath, which upon exhaling in colder climates appears visibly as a white mist. This belief may have also fostered the metaphorical meaning of "breath" in certain languages, such as the Latin spiritus and the Greek pneuma, which by analogy became extended to mean the soul. In the Bible, God is depicted as animating Adam with a breath.
In many traditional accounts, ghosts were often thought to be deceased people looking for vengeance, or imprisoned on earth for bad things they did during life. The appearance of a ghost has often been regarded as an omen or portent of death. Seeing one's own ghostly double or "fetch" is a related omen of death.
White ladies were reported to appear in many rural areas, and supposed to have died tragically or suffered trauma in life. White Lady legends are found around the world. Common to many of them is the theme of losing or being betrayed by a husband or fiancé. They are often associated with an individual family line or regarded as a harbinger of death similar to a banshee.
Legends of ghost ships have existed since the 18th century; most notable of these is the Flying Dutchman. This theme has been used in literature in The Rime of the Ancient Mariner by Coleridge.
Source: Wikipedia, "Ghost", available under the CC-BY-SA License.