Surely if a person appears in front of us, then disappears mysteriously, or goes "Wooo!" or whatever, we're quite likely to accept we have seen a ghost. However, if an inanimate object (table, tardis, cheesegrater, whatever) does the same, we're more likely to apply the word 'hallucination' to what we have witnessed.
I'd say that if the sorts of things going on which make people see ghosts, that is, 'people ghosts', (tormented undead spirits? Psychic echoes? Sub-sonic vibrations? Ergotism?) are the same sorts of things that cause people to perhaps see 'not-people ghosts', then we have a situation where two or three different words, with very different implications attached, are getting used:
1) If I see a person dissappear through a wall, I've 'seen a ghost' - I will therefore assume that there is a mystery which I have witnessed which may involve the undead/ the enernal soul.
2) If I see a table float through a wall, I could be 'hallucinating' - I will therefore assume that there is something physically or mentally wrong with me, which has caused my senses to be tricked.
3) If I see a large lake in an area of desert, and run towards it only to find it ain't there, I've seen a 'mirage' - I will therefore assume that there is something special about the environment I'm in which has caused my sense data to be confused.
So, we can use loads of different words to describe the phenomenon of seeing stuff that 'isn't there' or isn't tangible in the same way that we are, but when the stuff in question appears to be a person, the word we use carries lots of implications about souls, afterlives, etc (and keeps Derek Acorah in business).
Hope that made some kind of sense.