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)

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)