Fixed Point in Time
A fixed point in time, or "fixed point" is a an event which cannot be changed via Time Travel. Usually this also means that the universe will endeavor to ensure that this event happens even when someone does try to change it. Some time travelers with ties to the 4th and/or 5th dimensions can sense when an event is a fixed point, such as the Time Lords. Some fixed points are also guarded to ensure that they aren't meddled with, for instance, "Hitler duty."
Known fixed points include: