Fixed point in time

Fixed points in Time (as distinguished from unfixed points in Time) were junctions at which history could change. A new timeline could spring from unfixed points in Time (DW: Cold Blood)

Definition
Fixed points were events and/or individuals who have such a long-standing impact on the timeline that no one, not even Time Lords, were allowed to interfere with their natural progression. The Doctor, while free to interfere in alien invasions and save planets in most cases, cannot interfere/interact with these fixed points. Examples included Jack Harkness after his rejuvenation by Rose Tyler (DW: Utopia (although the Doctor nonetheless shared several adventures with him), the destruction of Pompeii by the Vesuvius volcano (DW: The Fires of Pompeii, and the death of explorer Adelaide Brooke (DW: The Waters of Mars).

The case of Brooke marks one of the only times the Doctor intentionally interfered with a fixed point, under the rationale that, as the last surviving Time Lord, the Laws of Time were his to command. Ultimately, he was unable to prevent Brooke from committing suicide, thereby allowing the timeline to unfold, with only a few minor changes. (DW: The Waters of Mars)