A mind probe was a device which allowed an operator to extract information from a subject's mind through the use of some kind of projected energy.
Subjects were said to be at risk of brain damage if they were exposed to these devices for either long periods or high power outputs.
The Third Doctor once told Jo that he had found a way to defeat mind probes, and quipped that he'd once destroyed a government's entire supply of probes. Later, when hooked up to an Earth Security mind probe, he demonstrated that his brain could destroy their versions as well. (TV: Frontier in Space)
A mind probe of Gallifreyan design was going to be used on the Castellan to discover the truth about the Black Scrolls of Rassilon, but he was shot before they could use it on him. Nevertheless, he exhibited great fear of the mind probe, which perhaps hinted at its power against the Time Lord brain. (TV: The Five Doctors)
Torchwood Three also had a mind probe. They used it on Beth Halloran when they were trying to discover whether she had attacked the burglars who broke into her house. Ianto remarked that the last alien they used it on exploded. (TV: Sleeper) It is unknown if the mind probe survived the bombing of the Hub when the British government attempted to destroy Torchwood. (TV: Children of Earth: Day One)