NOVA bomb

First mentioned in Halo: First Strike, the NOVA is a special device that uses nine fusion warheads in a ring formation. Once detonation occurs, the lithium-triteride fusion cores combine together and the resulting blast is multiplied one hundred-fold. Nick-named "planet-killers," these have enough explosive power to destroy an entire planet.

In the novel, they were left on Reach and "repurposed" by Admiral Whitcomb. His reasoning was that "either the Covenant pack those bombs up and take them home for study, a possibility that I hope to God happens. A bomb like that could crack their homeworld in two.  Or, the bombs stay here, and they'll stop the Covenant on Reach..."

The resulting explosion detonated between the only known Covenant planet Joyous Exultation and its moon, Malhiem, scorching a quarter of the planet and shattering the moon, and the resulting winds on Joyous Exultation flattened cities and spawned tidal waves.

Delta Halo
In Ghosts of Onyx, Cortana requests that the UNSC launch a NOVA Bomb around Delta Halo and High Charity to stop the spread of the Flood and destroy the Covenant Holy Capital.

Trivia
The Nova Bomb was a bomb from the book Starship Troopers, wherein it is described as "being able to split a planet in half." It is likely that this inspired the Nova Bomb.