Causal nexus
A causal nexus, also called a time-space nexus, was the connection between a cause and its effect, i.e. causality and to an extent continuity.
Generally, a causal nexus, or, more specifically, a causal nexus point or temporal nexus point, was mentioned only if there were an anomaly or the potential for an anomaly. A nexus could exist in time, space, or both, and so were consistent with universe models that included a space-time continuum. While discussing changing her personal past, Older Amy Pond capped a list comprised of destiny and causality with the nexus of time itself, a structure of causal nexus points that described or defined time. However, at the time she was referencing changing her own recent personal history from within a stable temporal anomaly that was generated artificially. (TV: The Girl Who Waited)
Causal nexuses were not extensively discussed, but as a structure of time they would be important to events creating time fissures, time tracks, and alternate timelines.
When asked why he couldn't just take the TARDIS back to the previous day, the Tenth Doctor recited, as if by rote, "I can't go back within my own timeline. I have to stay relative to the Saxon Master within the causal nexus." He earlier called the same set of events a convergence. (TV: The End of Time)
In time
Time fissures possessed nexus points that could be crossed from one reality into another. Temporal nexus points were not visable, but could be sensed by time sensitives. (AUDIO: The Wreck of the Titan)
The Doctor's TARDIS became caught in a spatio-temporal causal nexus point when it hit a bump in the time track, generated by the Daleks of an alternate timeline using a time corridor to pull the Fifth Doctor into their future. (AUDIO: The Mutant Phase)
The Fourth Doctor had stated that every point in time had its alternative, (TV: Pyramids of Mars) but some causal nexuses might have provided a breaking point for more stable timelines off the main universe.
The causal nexus divided when Nyssa changed history, causing the universe to exist in two quantum states and releasing blinovitch energy. When the time paradox was corrected with another paradox, the Fifth Doctor became the causal nexus as the paradox resulted from knowledge he would claim not to know in his personal future. (AUDIO: Prisoners of Fate)
In keeping with this, the "Wenley Moor Affair" was a crucial nexus-point in Earth's time stream because countless futures could diverge from that moment into alternate realities. (COMIC: Final Genesis)
Rose Tyler, having travelled to Donna's World, refused to reveal her name. She stated that "one word in the wrong place can change an entire causal nexus." When she whispered the words "Bad Wolf" in the alternate Donna Noble's ear, it was only at the point that that Donna was about to die, awakening the original Donna that was being fed on by the time beetle on Shan Shen. (TV: Turn Left)
In space
Causal nexuses seemed to exist as actual structures of the universe. They could be described based on planets. Earth was widely recognised as one of the causal nexus points of the Mutter's Spiral. (PROSE: Cat's Cradle: Witch Mark)
Logopolis and its inhabitants ran the Charged Vacuum Emboitments that kept N-Space going past the point of collapse. When the Tremas Master stopped Logopolis, he caused the unravelling of the whole causal nexus. This released a wave of entropy that engulfed parts of the universe, killing them. (TV: Logopolis)
A nexus resided in the transjovian space near Jupiter, which the Hand of Omega reached using faster-than-light travel, after which traversing the nexus to reach Skaro in its own time zone. (PROSE: Remembrance of the Daleks)
In both
The Seventh Doctor considered (and blamed) Earth as a "major time-space nexus" for the reason he was on Earth at certain times and the coincidences that came from that. These included him being in Mesopotamia in the time of Gilgamesh and being in Ife during the 10th century. In the latter travel, there may have been static electricity involved which made the Yoruba believe he was Shango, the thunder god. (PROSE: Transit)
The Edifice's interior dimensions were mapped onto its exterior, making it the same size inside as outside, cancelling its dimensional transcendence. This also caused it to become a nexus point, affecting past and future events along the causal pathways, generating temporal anomalies in the resulting temporal pulses. (PROSE: The Ancestor Cell)
The particles of the Time Vortex could clump together at a nexus point. A TARDIS could hold onto a nexus point with mathematical anchors to resist the streaming delta flows. (PROSE: Cat's Cradle: Witch Mark)
