The answer to the title question is easy: when someone looks at a fuzzy blob in a picture and says it's a ghost.
Take a look at the picture taken by people visiting the site of a 363 year old battle:
The picture, credited to the Northampton Paranomal Society, is claimed to show a soldier from that battle.
The money quote from the article is this:
“When we saw it, when we were looking back through the pictures, we were gobsmacked.
“We’re saying that it’s a soldier. Some people can see it sitting on a horse and some people just see it as a walking soldier.”
Yes, the picture is so clear they can't tell if it's a human being or a horse!
And that's from the leader of the paranormal society.
Still, it looks more like a soldier than a tub of spumoni looks like Jesus:
I circled it to make it more obvious. Jesus? I see General Zod. Or Emperor Ming. Or the old school Doctor Who nemesis The Master.
Seriously, if two dots and a curved line can be seen as a smiley face -- and it's impossible not to see a smiley face -- then slightly more complicated randomness can be seen as either a man or a horse... if you wish hard enough.
So I think all these apparitions are nothing more than wishful thinking. A ghost, a horse, Jesus, Mary, even General Zod, are all products of our minds trying to make patterns out of random noise.
That's my claim, and I've steaked it.
Tip o' the ectoplasmic residue to Dave Pearson for the ghost story, and to the several folks who sent me the scoop on the spumoni.