Causal nexus

A causal nexus, also called a causal nexus point, time-space nexus, temporal nexus point or other variation, was a feature of non-linear temporal cause and effect that could be anomalous. They could exist in both time and space, and so were consistent with universe models that included a space-time continuum. There was no real detail or explanation as to the nature of causal nexuses, but it was connected to time fissures and likely related to time tracks and/or 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 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 Doctor had stated that every point in time has its alternative (TV: Pyramids of Mars), but causal nexuses may be the breaking point for more stable timelines off the main universe.

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, probably 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 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)

In both
The 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)