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 a Covenant outpost 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. This was actually extremely unfortunate as the Elites, lead by Imperial Admiral Xytan 'Jar Wattinree could have shattered the Brute fleet, thus effectively destroying the Covenant. This could have turned the tide of the war for both the UNSC and the Covenant Separatists.

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.
 * A Nova Bomb is also the common name for the heavy bomb used in Starfox games. Though not nearly as powerful as the Halo and Starship Trooper bombs of the same name, the Nova Bomb is one of the most powerful weapons you can use in the Starfox series.
 * The Nova Bomb also appears in the Andromeda TV series. In that show it is a weapon capable of causing stars to go into the Supernovea stage, therefore destroying the system around it.