Among the many definitions of "ghost" on Dictionary.com, the listing from American Heritage Dictionary includes:
1. The spirit of a dead person, especially one believed to appear in bodily likeness to living persons or to haunt former habitats.
2. A demon or spirit.
A WordNet search comes up with
1. The visible disembodied soul of a dead person.
2. Shade, spook, wraith, specter, spectre (a mental representation of some haunting experience)
While About.com offers the following lengthy definition:
A ghost - or spirit or apparition - is the energy, soul or personality of a person who has died and has somehow gotten stuck between this plane of existence and the next. Most researchers believe that these spirits do not know they are dead. Very often they have died under traumatic, unusual or highly emotional circumstances. Ghosts can be perceived by the living in a number of ways: through sight (apparitions), sound (voices), smell (fragrances and odors), touch - and sometimes they can just be sensed.
The last time Gallup asked Americans if they believed in ghosts, one third said yes.
In a June 6-8  poll, 37% of Americans told Gallup they believe in haunted houses, while 46% say they don't, and 16% aren't sure.
About three in four Americans profess at least one paranormal belief, according to a recent Gallup survey. The most popular is extrasensory perception (ESP), mentioned by 41%, followed closely by belief in haunted houses (37%).
Ghosts/that spirits of dead people can come back in certain places/situations - 32%
A special analysis of the data shows that 73% of Americans believe in at least one of...10 [paranormal] items..., while 27% believe in none of them. A Gallup survey in 2001 provided similar results -- 76% professed belief in at least one of the 10.
Indeed, I think most people would agree that ghosts, or at least the idea of them, have been haunting we humans since the dawn of our existence -- and probably will not disappear any time soon.
What do you think a ghost is? Have you ever seen one?
Gallup - Who's There? (Video)
About.com - What is a ghost?
Wordnet - Ghost
Dictionary.com - Ghost