Deuterium burning (or deuterium fusion) deuterium nuclei. Parts of proton-proton chain fusion include this as a step, and a body that contains deuterium can trigger it if sufficiently hot. It requires a lower temperature than does hydrogen burning.
The defining line between giant planet and brown dwarf is that the latter is sufficiently massive to generate the heat to trigger deuterium burning. This can happen if there is deuterium to burn, but there will only be enough to sustain fusion for a brief portion of its life.