Ghost

Ghosts were a phenomenon occurring in collective mythology and popular culture. Those who believed in ghosts sometimes held them to be souls that could not find rest after death, and so lingered on in the world of the living. This inability to find rest was often explained as unfinished business.

A ghost was 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. This anomaly leaked images and other phenomena from light years away onto Earth (DW: Image of the Fendahl)

The Ninth Doctor later matched this explanation while fighting the Gelth. (DW: The Unquiet Dead)

The quantum transducer was an extradimensional artifact that amplified residual hauntings into empathic visions. These ghosts were explained as psychic residue from powerful emotional events. Thus the transducer was dubbed the "Ghost Machine". (TW: Ghost Machine)

Eugene Jones was able to interact with Gwen Cooper of Torchwood Three for a time after his death as a de facto ghost. Ultimately, he briefly became visible to not only Cooper, but to everyone, before "crossing over" and ceasing to be a ghost. This was due to his consumption of a Dogon Sixth Eye beforehand granting him a new perspective on life. He would eventually unlock the secret to true peace and happiness allowing him to let go. (TW: Random Shoes)

A Cardiff building was haunted by three cowled men with scythes, it was unknown what these things actually were, but they were considered ghosts. Toshiko Sato and Ianto Jones were sent by Torchwood Three to kill them. They appeared in front of them, and were shot dead, revealed as just "shadows" cast by the Rift. (TW: Exit Wounds)

The Eleventh Doctor explained that psychic residue was a cause of many ghost sightings. (DW: The Pandorica Opens)

Sarah Jane explained that events get recorded on their surroundings and under certain circumstances can get played back again. (SJA: Eye of the Gorgon)

The "ghosts" encountered at Ashen Hill Manor were actually people who had been trapped in-between the dimensions by Erasmus Darkening's malfunctioning equipment. (SJA: The Eternity Trap)

When Sarah Jane was sent back in time to collect a fragment of Chronosteel, she encountered a ripple from the future, echoes of a traumatic moment that had yet to happen. Her actions helped to prevent the event from occuring. (SJA: Lost in Time)

Sergeant Benton once confronted specters, the tortured souls of his military father and child brother, who had both tragically lost their lives, holding him in contempt, to the grave and beyond. Now facing up to his traumatic past, Benton was lost in a psychotic fantasy world, a sadistic game devised by his forebearer and sibling, to teach him a lesson. Finally set free, he was at last able to get back to his post where he was most desperately needed. (RP: Wartime)

Professor Alistair Gryffen's numerous failed attempts to retrieve his family with the Space-Time Manipulator eventually began to take their toll on the continuum, warping reality into the shapes of his wife and children. They functioned as de facto ghosts, needing to trade places with Jorjie Turner and Darius Pike so they could take physical form, even going so far as to restore the Professor to his default setting, as he was before he lost them. They were ultimately banished back into nothingness. (K9TV: The Fall of the House of Gryffen)

Mistakenly identified as ghosts
The Cybermen were once mistaken for an army of ghosts when crossing over from a parallel universe. (DW: Army of Ghosts, Doomsday) The Third 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 a translucent blue. (DW: The Unquiet Dead) In Arcopolis, the Eyeless were mistaken for ghosts. (NSA: The Eyeless)