Time-travel

Time-travel is the fictional ability to "jump" from one point in time to another rather than simply progressing forward through time in the normal manner. The ability to move backward through time, visiting a previous point in history, is the most blatant expression of this phenomenon, although the ability to jump into the future is also common.

Time-travel introduces many potential plot holes into a story. For instance, it opens the question of whether a time traveler can alter history. The classic example of the problem is the "Grandfather Paradox": if you travel back in time and kill your grandfather before he conceives your father, thus preventing your own birth, how can you exist to go back in time?

While some time-travel stories treat history as immutable -- the time-traveler failing to make the desired change in history or even becoming the cause of the event s/he wanted to prevent -- others allow alterations in history, sometimes resulting in parallel universes.