Ghost hacking is a fictional activity depicted in Ghost in the Shell, whereby a person -- or machine -- uses similar technologies to hacking a computer, in order to gain direct access to the conscious mind of another person.
In the Ghost in the Shell universe, people have ways in which they can connect their brain directly to diverse networks. While people generally use a firewall, a good hacker or cracker can bypass it and gain direct access to the mind of the person in question.
A person who has ghost hacks can do a diverse number of actions with a victim. Victims can have their senses tapped into by a ghost hacker, allowing the hacker to know the information the victim's physical senses are relaying, while a more extreme hacker can overwrite the victim's memories and thoughts to completely manipulate their actions.
In most cases, victims do not know they have been ghost hacked unless told by an outside source.