A ghost is often depicted as a transparent human-like creature.

Some believe that a ghost is the immortal remains of a human being. Ghosts - or apparitions - are classic.
There are numerous accounts of ghosts, as well as pictures, videos and other alleged evidences.
However, it has never been possible to induce a ghost, at laboratory environments.

Science does not know what a ghost is made ​​of and how it apparently can have solid properties, and despite go through for example walls.
A ghost can be a diffuse floating mist or even a creature of almost flesh and blood.

There are many theories what “ghost-matter” is. Some theorize subatomic neutrinos, dark matter or ectoplasm.

It is not clear, if a ghost can communicate or not. This differs from case to case. A ghost can sometimes manipulate its surroundings – or even hide objects. There is no clear border between the term ghost and poltergeist. It is said that a ghost is more subtle, as a poltergeist is much more violent, noisy and threatening.  Some differentiate a ghost of being the remnant of a dead soul, while the poltergeist is demonic.

A truly recognized proof of a ghost does not exist. As such, a ghost can not be measured or defined. Many try to confirm a presence of a ghost through EMF meters, photographs, heat pictures or spiritual sensing. However, it does not change the fact, that no recognized link has been found between the phenomenon and the measurement method. Often stories go, that a witness is quite sure to have seen a ghost. But the witness can not explain why. We can theorize that a ghost can reflect the observer's background, hope, fears, etc.

However - ghosts have been reported long time before TV and CGI. So maybe a ghost is quite real!

 

Kobberstik,
Engraving, "The Hammersmith Ghost", 1834. Even before CGI and picture manipulation, ghosts have been reported.