An illegal practice whereby two or more market makers collectively attempt to
influence and change the price of a stock. Ghosting is used by corrupt
companies to affect stock prices so they can profit from the price movement.
This practice is illegal because market makers are required by law to act in
competition with each other. It is known as "ghosting" because, like
a spectral image or a ghost, this collusion among market makers is difficult to
detect. In developed markets, the consequences of ghosting can be severe.
Source: extracted from a stolen note book of a rocket scientist
(come on .. everybody knows its from investopedia )