A time ghost is a type of apparition phenomena caused by a temporal disruption or some other disturbance in the normal passage of time.
History and specifics
In the year 2372, when a tipsy Jadzia Dax was disturbed by an unexpected noise, she was ready to dismiss the occurence as a side effect of her inebriation from alcohol. However, she mused that Starbase Deep Space 9 had frequently played host to prophetic visions, subspace disruptions and time ghosts, often enough that she might actually be experiencing something beyond a simple auditory hallucination. (DS9 novel: Revenant)