The dimension cannon or the dimension jump was a device that could travel between dimensions, at the cost of ripping a hole in the universe. In normal circumstances, it didn't work, it only worked if the walls of the universe were broken. It was created in Pete's World by Torchwood. (TV: Doomsday, Journey's End)
First version (Battle of Canary Wharf) Edit
After the Cybermen left Pete's World to travel to the Doctor's universe, Torchwood created a device that could hop to another universe. This first version resembled a yellow disc. It would hang by someone's neck like a necklace and could teleport at the same position on the chosen world where they teleported from the other world. The use of the dimension cannon opened the breach further and further and raised the temperature on Pete's World. Pete's version was capable of transporting an entire group of people wearing the medallions in the same area while some (like Rose's) could only teleport the single user. After the Tenth Doctor sucked the Cybermen and the Daleks in the Void, the breach closed itself because of the amount of Void stuff present. The devices stopped working as a result. (TV: Doomsday)
Second version (War in the Medusa Cascade) Edit
This new version, unlike the original, was handheld and could travel to different parts in space, but it needed to recharge for thirty minutes after each use. The dimension cannon could also measure timelines.
Mickey and Jackie used it to save Sarah Jane Smith from a pair of Daleks following their invasion of Earth. Jackie also used it to escape from the testing of the reality bomb.
Following the invasion and Davros being prevented from using the reality bomb in the Doctor's universe, the walls of the universe eventually started closing themselves, a process that the DoctorDonna called a dimensional retroclosure. (TV: Journey's End)