Causal nexus

A causal nexus, also called a causal nexus point, time-space nexus or other variation, was an anomaly that could possibly exist in both time and space. There was no real detail or explanation as to the nature of causal nexuses, but it was likely related to time tracks and/or alternate timelines.

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. (NA: 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 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. (DW: Logopolis)

In Time
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. (BFA: The Mutant Phase)
 * In this sense, causal nexuses may be the breaking point for parallel universes off the main universe.

This was supported by Rose Tyler who, 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. (DW: Turn Left)

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. (NA: 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. (EDA: The Ancestor Cell)