Time ram
Talk0A time ram was what happened if two TARDISes attempted to occupy exactly the same time and space.
The Third Doctor threatened to send the Master into time ram - even though it would kill them both and Jo Grant - to prevent him escaping with the Crystal of Kronos. The Master called his bluff, realising he wouldn't kill Jo. However, Jo used the controls herself to cause a time ram. They were all saved by Kronos, who brought them into its dimension. (TV: The Time Monster)
According to the Fifth Doctor, a time ram would cause a black hole large enough to destroy an entire universe. (TV: Time Crash)
It could be narrowly avoided by a TARDIS materialising within another. (TV: The Time Monster, Logopolis, Time Crash)
The Sixth Doctor performed a time ram against the remains of Iris Wildthyme's TARDIS at the centre of a dimensional nexus. Local conditions limited the effects to merely eliminating some possibilities from reality, but the force was still enough to destroy the rest of Iris' TARDIS. (AUDIO: The Wormery)
The Seventh Doctor attempted to perform a time ram against the Timewyrm in order to destroy her, but she escaped into the Time Vortex. (PROSE: Timewyrm: Genesys)