Time ram

Time ram was the name given to what happened if two TARDISes attemptted 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 an Jo Grant - in order 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, causing a time ram to occur. They were all saved by Kronos who brought them into its dimension. (DW: The Time Monster)

According to the Fifth Doctor, a time ram would cause a black hole strong enough to destroy an entire universe. (DW: Time Crash)

It be narrowly avoided by the two TARDISes materialising within each other. (DW: The Time Monster, Logopolis, Time Crash)

The Sixth Doctor performed a time ram against the remains of Iris Wildthyme's TARDIS at the center 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. (BFA: The Wormery)