The general definition is the discooperated anima of a living being that for some reason lingers after the death of that person.
When looking for Ghosts it has been found that the Littleton Uncertainty Principle holds true. The more technology you use the harder it is to find them.
Categories of Ghost
These are the least ghost you can find. In general they are seen, never heard and cannot be interacted with. Apparitions can be anything from a cold spot in an other wise warm room, a face in a window, or even the reenactment of a crime. Full body clear as day photos are rarer than hens teeth. The Littleton Uncertainty Principle is believed responsible.
These sort of ghosts do not have a mind behind them. They are more heavily imprinted psychic images than full anima. They are commonly associated with an event of terrible passion that resulted in the death of the now apparition. They are psychic movies. They play out their events and cannot be interacted with.
Clearing Apparitions requires a through spiritual cleansing of the area they affect. Exorcism followed by a good blessing will usually do the trick. The bad must be cleaned out and the good invited back in. If this fails, you didn't have a cleric that knows their stuff.
In truth many people that find they have Apparitions choose to live with them. Once they know that the Apparition is not harmful they often see it as a psychic pet and a point of bragging rights on their "haunted house". Some people's children.
The Shade has more to it. Shades will be heard, not just seen. The footsteps on the stair, the image of a person just out of perception. You still get cold spots and such but the element of sound is present. You often hear them more than see them. Sometimes addressing the Shade will get a reaction. A sound, or the ending of said sound. Shades are not generally harmful, except for the disquiet they produce in the living. The majority of hauntings will be Shades.
There is more than violent passion involved in the creation of a shade. There must be a reason. Not all Shades are born of murder. They all are however, born of need. The person the Shade was died with a terrible need that weighed down on their soul. They must deal with this need before they can move on.
Most Shades will communicate. In general it is only about that terrible need and how it must be dealt with. They do not possess a complete intellect, only a hollow unfulfilled need that defines them.
Laying these spirits is more work. Even a exorcism and blessing will not permanently move them on as long as that aching need remains. It is necessary to identify the need and communicate its end. A more stubborn spirit, like McKinley may require some holy admonishment. Have it handy.
Most people do not want these spirits around. They are bothersome. This is a case where good Mediums are an advantage as they can generally get the Shade to communicate the need and be communicated with.
This term has been knocked around for everything for a restless spirit to teenaged psychokinetics unaware of what they are doing. they have also been easily conflated with the Polterghost (see below). In the Bureau lexicon this refers to a Shade with a more solid existence. A Poltergeist can move things. Anything from banging doors, moving furniture, even flinging things across the room. One known example in Marietta Ga. is the late owner of a hardware store that still neatens the shelves and will aid customers. Being the spirit is benign the current owners have left it be.
Not all Poltergeists are benign. Some will try and hurt the living from throwing things to actual physical battery. These are undead that are with it enough to be a physical danger to the living, and we recommend laying them. Like Shades Poltergeists can be communicated with. As the urgent need is still present it will dominate any communications. If it does not, you are dealing with a Polterghost, banish accordingly.
Putting down the Poltergeist is similar to dealing with a Shade with an added physical danger if they decide they don't want to move on. We recommend some heavy holy help when this comes up.
The vast majority of people will cooperate with the removal of Poltergeists. The hardware store owner is an extremely rare example.
Wraith and Specter
Previously we have dealt with restless dead that were mainly restless. Now we move to those that are actively evil. The Wraith and Specter, really the same creature at different levels of power. This is an undead spirit that has an active hate of the living. They are possessed of a full intellect and can be communicated with, if they bother. These are not merely the spirits of the bothersome or even douchebags. We are talking the truly vile that failed to hang onto their body like a lich. Hitlers, Pol Pots, that kind of evil. On the good side they are generally tied to a location can cannot move about at will.
These can hurt you. Their touch is physically painful. It actually will damage your anima. They are hard to hit with physical things. They must themselves become slightly solid to attack and that will allow a chance at hurting them. Greyhawke has a magic property they call "ghosttouch" added to magical weapons designed to damage incorporeal creatures. The Blackmane force pistol is one such item. Each team should have one.
Also on the good side, once destroyed they are gone. They do not regenerate as do some undead.
No one wants these save more powerful and vile undead that wish them as guardians. Blast early, blast often.
The true Ghost is one of the rarest, therefore most talked about phenomenons. They are the result of trauma or deep need, but unlike the Shade are fully cognizant of their condition. This need is not always, or even frequently benign. Ghosts can have physical elements, bloody trails, moving objects, a whole cacophony of sound. Evil Ghosts are far more common than those that are not, and they have a number of ways they can damage the living, if they so choose.
Ghosts can harm by touch, sound, or even appearance. If you even suspect hostile Ghosts go prepared, negative plane protection will mitigate their attacks. The horrific appearance has been known to age the living suddenly and dramatically. The chilling touch of a ghost can harm your anima. Their moans and screams will sap your ability to function.
While Shades are hanging around due to a need, Ghosts are generally held. Something not done in life, how they died, or their own vile actions have pinned part of their very soul to the spot they haunt. This makes a Ghost extremely hard to lay. Blasting them out of existence while possible does not get rid of them, they regenerate. It is necessary to find and relieve the condition that holds them to the Earth. Be that a body unburied, a morbid attachment to earthly wealth, or even a fear of their eternal fate. Bring heavy holy help. Some Ghosts cannot be helped, and need to be destroyed, while hard it is possible, but only with divine aid.
Sane people do not want these beings around. Deal with them.
Things that are commonly conflated with ghosts but are not.
These incorporeal spirits are actually demons of a very minor nature. They are more bother than threat and are easily dealt with under most circumstances.
The Shadow looks like just that. A Shadow that has nothing producing it. They are creatures of the Plane of Shadows. They are noted for feeding on the anima of the living and therefor a threat. They are true incorporeal forms and require ghostouch weapons.
Will o Wisp
Also called friar's lantern, hinkypunk, corpse light, and other names depending on region. These are evil incorporeal fay creatures. They feed off the anima of the dying and so try to lure people into places of danger, usually swamps. Animals are not enough, they want the sentient. Once again Ghosttouch weapons are a must.