A temporal cicatrix was a scar where spacetime had tried and failed to heal itself. (PROSE: Unnatural History) It produced an area of temporal anomaly, (PROSE: The Gallifrey Chronicles) causing effects like freak weather patterns and sudden gusts of gravity or the reverse.
The Eighth Doctor accidentally created a cicatrix in San Francisco that was responsible for the Millennium Effect. It attracted to San Francisco several creatures that travelled through higher dimensions, including the Kraken. Daniel Joyce's Advanced Research Project used the scar to test their mathematics. (PROSE: Unnatural History)
A cicatrix was formed in the Shoal when a naked singularity briefly appeared there. Over the next three hundred years, the Vore flocked to the cicatrix, since they were instinctively attracted to it; they learned to ride the hyperspace corridors formed naturally by the scar. Ulysses, Penelope Gate, Saldaamir, and Larna studied the cicatrix for their project. When Marnal destroyed the cicatrix, the Matrix projected that the Vores' history had changed to become a threat to Gallifrey, making them a potential Last Contact. (PROSE: The Gallifrey Chronicles)