Ghost

Ghosts are a phenomenon occurring in the collective mythology and popular culture of Earth and other worlds including, perhaps, Skaro. Those who believe in ghosts sometimes hold them to be souls that could not find rest after death, and so linger on in the world of the living. This inability to find rest is often explained as unfinished business.

A ghost is sometimes referred to as a fetch, and the woods around Fetch Priory were long believed to be haunted, the result of its being near a relative continual displacement zone, or hole in time. (DW: Image of the Fendahl)

When the Torchwood Institute began its studies of the breach created by the Void Sphere's entry into our universe, their attempts to harness the energy seeping through the breach drew in translucent, vaguely humanoid forms which were sighted all over the world and quickly identified by those who saw them as ghosts. Some, like Jackie Tyler, believed them to be the spirits of dead family members. A psychic link developed which the ghosts used to strengthen their presence in this world. Each time the Torchwood technicians drew energy through the breech, the wall separating realities grew weaker and the ghosts grew stronger and more solid. The Doctor realized too late that the other world these ghosts originated from was Pete's World, and that the ghosts were in fact the race of Cybermen from that alternate reality which he had encountered before. Once the Cybermen seized control of Torchwood, they used the institute's equipment to open the breach and usher an army of five million "ghosts," which quickly achieved corporeal form as Cybermen and began a ruthless takeover of the planet. (DW: Army of Ghosts)

Minor references

 * The Doctor and Sarah Jane Smith were once mistaken for ghosts. (MA: The Ghosts of N-Space)
 * The Gelth were mistaken for ghosts, as their gaseous forms could fly and were blue transparent. (DW: The Unquiet Dead)