This is the Skeptic's definition:
http://www.skepdic.com/ghosts.html
It is based on the believer's definitions of what ghosts are. The problem is that ghosts (the real thing) are not disembodied spirits of the dead who linger in this world to ...blah, blah, blah.
Ghosts fall into a gray area of psychosomatic responses resulting to exposure to unknown external forces. Ghosts are very real to the person who sees or experiences one in that they can feel them, hear them, and interact with them. At the moment they have the experience there is no distinction between reality, and perceived reality.
The chief culprit is Infrasound:
http://www.skepdic.com/infrasound.html
If you are looking for repeatable evidence then Infrasound is the 500-pound gorilla behind most ghost sightings. The best evidence for this was the rise of reported hauntings in the 1990s was directly related to the change in basic home construction from modest-sized homes to the McMansions with greater floor space, and higher ceilings. Picture a building being like a trombone, the more room air has to circulate the lower the pitch, and in the case of a larger home the greater chance of Infrasound causing people to believe their NEW house had a ghost. But since Infrasound is not well known, people don't even think about it. Many of these hauntings could be eliminated just by deadening the walls with a tapestry or decorative curtain to eliminate the frequency.
Infrasound bears out well because if you look at the types of spirits that are seen by people in different geological locations (forests, flat-deserts, mountainous deserts, coastal regions, areas covered by snow or ice, and cities with tall buildings) you see the same types of phantoms, or paranormal activity being experienced by people with no proclivity to believe in such things. The question is why? The answer is that different types of Infrasound causes specific types of psychosomatic responses. One only has to look at the types of gods worshiped by primitive peoples living in forests, and compare them to those of desert dwellers. Forest people have multiple gods, and desert people tend to have fewer god, or even one god.
Infrasound is caused by shifting seismic plates, air moving through trees, the sound of sand grains being blown around, and air circulating in a large confined space.
The other main culprit behind ghosts is bad electrical wiring. Your body has an electrical field, and sometimes there is a conflict with a competing field generated from a device, or internal wiring. Think about all those old homes rumored to be haunted, and in most cases the wiring was done over 50 years ago, and then improved ad hoc as demands for power changed over time.
If any of you guys have ever worked on the old cathode-ray tube (CRT) you know there is a danger from being around it when the housing is removed, and the charge improperly bled out. Guys have blacked out, lost memory, and even heard voices as a result.
So when I say ghosts are real I mean the experience is 100% real to the person who has the encounter. They will give you detailed descriptions, that are consistent, and vivid. While the encounter takes place almost exclusively inside the head, there is no way for the person to know.
In this case skeptics are both right and wrong.
For instance, the most common ghost is of a living person. That undermines the Victorian-era definition of a troubled spirit all by itself. My guess is that many of you have had the experience of hearing a family member in the other part of the house, but later discovered they were somewhere else. There is a good chance you DID hear them, but you rationalized it away as a mental mistake. Yet at the time you were certain they were present.
The other big cause of paranormal activity comes from CO2 from bad furnaces or heaters. When someone tells me they think their house is haunted the first thing I ask is if they've had their heating checked. (In California, home owners are required to install Smoke and CO2 detectors due to the rise of this problem).
Finally, in 2001 builders began to import dry wall from China for use in the South, and South Western U.S. Again, suddenly people who never believed in ghosts began to have experiences.
It is not a question of IF people have experiences with ghosts, but WHY. If it was only an internal psychosomatic event then why do different people see the same thing, or have similar encounters? I always ask: If it's in my head, then why don't I see Playboy playmates in my room at night? To be honest, that's pretty much what occupies my subconsciousness, yet this has never happened to me or anyone one else. The fact that people is people rarely see what they
want to see, so why do they see the things that they do? The answer is Infrasound, or other rare phenomenon.
And no, not everyone is affected. Just like not everyone can sing, or understand color well enough not to dress like an idiot.
Just my .02.