What Is A Ghost?
There are many theories out there concerning exactly what ghosts are. No theory currently explains all aspects witnessed concerning apparitions and hauntings in general. Amongst the popular theories are that ghosts are trapped spirits, a recorded image as in the stone tape theory or perhaps hallucinations of some sort induced by an environmental factor such as Electromagnetic Fields (EMFs).
After being involved in studies looking into EMFs at haunted locations I am inclined to believe they play a key role in hauntings generally but they can still be shoe horned into other theories, such as refering to them as spiritual energy.
I think personal bias will effect the way in which we all approach an investigation, therefore it may be useful to discuss some of the theories we champion, look at their strengths and weaknesses and discuss the best way to devise experiments around them.