The light-speed overdrive was a component of a TARDIS. It allowed signals to travel at faster-than-light speeds.
The Fourth Doctor and the Tremas Master used the light-speed overdrive from the Master's TARDIS to broadcast a program through the Pharos Project's radio transmitter to a CVE, stabilising it and keeping it open to shunt entropy out of the universe. (TV: Logopolis [+]Loading...["Logopolis (TV story)"])