The Nova device was a weapon of Movellan design which was capable of destroying an entire planet, or at least its atmosphere.
This device counted down to zero, at which point it altered the molecular structure of the atmosphere so that the atoms become flammable. Without protective shielding around the device, it would burn up an entire planetary atmosphere. It could, however, be turned on without being armed (making it into basically nothing more than an elaborate egg-timer).
During the Dalek-Movellan War, a Movellan force followed a Dalek group to their homeworld of Skaro where they sought to reawaken Davros in order to use him to program their battle computers to end the logical stalemate the Dalek Empire was facing against the Movellans.
When discovering their aim, the Movellans planned to use the Fourth Doctor to reprogram their own battle computers to defeat the Daleks. In order to deny the Daleks a similar advantage, they planned on detonating the nova device on Skaro thus destroying Davros and his creations but this was thwarted by the interference of the Doctor along with his companion. (TV: Destiny of the Daleks)