Ghosts are different things depending on who you ask. While 32% of Americans believe in ghosts, according to a 2005 Gallup poll, there is a divergence of opinion as to what constitutes a ghost and a large range of entities that may be called "ghosts" .
The dictionnary tells us that a ghost is the spirit of a dead person, especially one believed to appear in bodily likeness to living persons or to haunt former habitats.
Parapsychologists are in favor of the term, apparition, because it's purportedly more precise. An apparition is considered immaterial, but it can seem real and tangible.
There are other uses of the term refer to angels or demons, or even as a synonym for a living person's immaterial spirit, whether or not they're presenting themselves to others.