A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or a logical contradiction that is associated with the idea of time and time travel. Temporal paradoxes fall into two broad groups: consistency paradoxes exemplified by the grandfather paradox; and causal loops.
A causal loop is a paradox of time travel that occurs when a future event is the cause of a past event, which in turn is the cause of the future event. Both events then exist in spacetime, but their origin cannot be determined. A causal loop may involve an event, a person or object, or information. A causal loop is also known as a boot-strap paradox, predestination paradox or ontological paradox in fiction.
The consistency paradox or grandfather paradox occurs when a future event prevents the occurrence of a past event that was partly or entirely the cause of the future event, thereby preventing the future event from occurring, thus creating a contradiction. Consistency paradoxes occur whenever changing the past is possible.