In astrophysics, hydrogen burning (or hydrogen fusion) is fusion of
hydrogen nuclei. References to hydrogen burning often mean
proton-proton chain (PP-chain), but it may sometimes be intended to include
the CNO cycle, or potentially deuterium fusion (deuterium burning).
Common use of the term is to define the lower limit of a
main sequence star: that it is large enough to develop sufficient
temperature at the core to trigger 1H fusion
(for which PP-chain would be the first reaction to occur).
Outside of astrophysics, hydrogen burning is more likely to refers
to the chemical process: in the presence of oxygen and at a
sufficient temperature, they chemically combine to form water
molecules.