Time bridge

A time bridge was a form of time travel used by the Third Doctor. While stranded on 20th century Earth and working for UNIT, he experimented with his Time-Space Visualiser; in the presence of a spacecraft fragment saturated with omicron radiation, the Visualiser was able to create a time bridge connecting to 1934 Salutua, where the spacecraft exploded. However, the time bridge proved to be a serious drain on UNIT's power reserves, and after his trip to the past resulted in history being altered so that Earth was conquered by former actress Nancy Norton, the Doctor destroyed the bridge. (PROSE: The Eye of the Giant)

Omega's anti-matter organism served as a time bridge, transporting those it came into contact with into Omega's anti-matter universe, simultaneously converting them into anti-matter during transit. The First Doctor informed his second and third incarnations of this, asking them what a bridge was for. When the Third Doctor gave the correct answer ("crossing"), the First Doctor told them to "stop dilly-dallying and cross it!" (TV: The Three Doctors)