A ghost is a purported spirit of a dead individual.
Before it was discovered what the two-dimensional lifeforms really were, Commander William T. Riker referred to them as ghosts and said he didn't want to be tailed by them all the way to T'lli Beta. (TNG: "The Loss")
For a moment, the Ux-Mal criminals on Mab-Bu VI appeared to the crew of the USS Enterprise-D as ghosts or spirits, that took control over the body of the living. Worf stated that the Klingon name for such a spirit was "jat'yIn". (TNG: "Power Play")
Some lifeforms are so alien that they can appear ghost-like. Anaphasic lifeforms are long-lived entities that can only appear corporeal when attached to a particular object, or a person with an unusual biochemistry. The females of the Howard family were host to an anaphasic lifeform, who posed as the ghost of a 17th century Scotsman named Ronin, for centuries. (TNG: "Sub Rosa")
- See also: Non-corporeal species
Many species believed that part of a person lives on after death; examples include the Bajoran pagh or the Ocampa comra. These beliefs were often found in religion. (DS9: "Emissary", "In the Hands of the Prophets"; VOY: "Emanations") Troi sensed the Devidians in the form the crew of the Enterprise-D found them had died in terror and she sensed an echo of life, though they still required sustanence thereafter.
While searching for information about the creators of the aphasia virus, Kira Nerys believed she searched for ghosts when she learned that one of the creators, Dekon Elig, died on stardate 39355. (DS9: "Babel")
In many legends, ghosts have the ability to appear and disappear at will. In 1944 of the alternate timeline created by the Temporal Cold War going hot, Jonathan Archer and Alicia Travers, about to be captured by the Nazi-Na'kuhl alliance, were brought aboard Enterprise via transporter, a technology almost incomprehensible to Humans of that time. After Alicia's return to Earth, Carmine told her of a rumor he had heard - that, after being cornered by the enemy, she and Archer had vanished into thin air, "like ghosts". (ENT: "Storm Front, Part II")
Commander Sisko said ironically that Rao Vantika must had an accomplice who did the system breakdown because two of the bodies found on the Reyab were in the station's morgue and he ruled out the possibility that Vantika's ghost was wandering through Deep Space 9's halls. (DS9: "The Passenger")