Time ram

A time ram was what happened if two TARDISes attempted to occupy exactly the same time and space.

The Third Doctor threatened to send 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)

When Magnus Greel's Time cabinet was being drawn out of its zygma beam, the Fourth Doctor and Romana I time rammed it to prevent the creation of an alternate timeline and ensure Greel arrived in London. (PROSE: The Shadow of Weng-Chiang)

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)

The first eight Doctors time rammed their TARDISes against the Master's TARDIS in order to defeat him in a pocket dimension where he had framed them by using a conceptual bomb which was erasing the Doctor's timeline. (AUDIO: The Light at the End)