Causal nexus

A causal nexus, also called a causal nexus point, 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 unraveling 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 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, having traveled to Donna's World, refusing 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, throwing the real Donna back into the real world. (DW: Turn Left)

In both
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)