A paradoxical situation in which a time traveler causes, through actions in the past, the exclusion of the possibility of the time travel that allowed those actions to be taken.
The typical example is that of the grandfather paradox, wherein a time traveler goes back in time and kills his grandfather before his father is conceived. It is a paradox because if this occurs, he will never be born, and therefore never be able to travel back in time to kill his grandfather, thus allowing himself to be born. This example is one type of causality loop. (Wikipedia)