Fusion reactor

A Deuterium Fusion Core is a species of a nuclear fusion reactor used as a power source on UNSC warships.

Description
Based on the name, a deuterium fusion core probably combines deuterium nuclei (a light hydrogen isotope) into heavier elements, accompanied by a net gain of energy. Deuterium may be fused with itself, or with tritium, another hydrogen isotope. This latter reaction is the easiest to initiate.

Since they have not been seen on screen, it is not possible to meaningfully speculate on the technique a deuterium core uses to induce fusion. However, a UNSC warship is powered by just two of these devices, so it could be deduced that they are not easily portable. Though not explicitly stated, it may also be used to power other kinds of spacecraft, and could be related to the oft-seen fusion core.

Real World
Real-world fusion reactors currently require more power than they release, so are not yet an economical means of electricity generation. Promising approaches include doughnut-shaped tokamaks, Polywells and laser-driven inertial confinement fusion (ICF).