From Halopedia, the Halo wiki
The Bident missile is a heavy guided munition utilized by the UNSC Navy.
Capable of being used at long-range, Bident missiles used to augment a warship's firepower by supplementing their Magnetic Accelerator Cannon. Using a scaled-down version of a starship's fusion drive for propulsion, the Bident uses a nuclear-pumped X-Ray laser warhead as its explosive munition. However, the warhead is capable of being swapped-out with kinetic rod bundles or even fusion warheads for conducting planetary bombardment.
A bident is a spear with two prongs, similar to a pitchfork. In Greek mythology, the god of the underworld, Hades, famously carries a bident as opposed to the three-pronged trident wielded by his brother Poseidon.