Subject: What is the scientific explanation for ghost?

I'm doing a project in my physical chemistry class on the science of 
ghosts.  I'm aiming to give an explanation of what the scientific 
community believes an apparition to be.  I'm specifically interested in 
finding out if a ghost is some form of matter.  If so, then what type?  
Also, what about the environment may stimulate the appearance of this 
matter/apparition in terms of physical conditions. 

Re: What is the scientific explanation for ghost?