Hydrogen noun (symbol H, atomic number 1) a flammable colourless odourless gas which is the lightest of all known substances and by far the most abundant element in the universe.
ETYMOLOGY: 18c: from French hydrogène, from hydro- + Greek gennaein to produce.
As a fuel
Hydrogen and Oxygen are combined to form water (H2O) and this is used in fuel cells and split to provide Hydrogen gas or liquid for powering vehicles. The byproduct of burning hydrogen in an Internal combustion engine is Water (steam), so it is consided a green fuel. But the production of the Hydrogen and the storage uses a lot of energy, To store as a liquid it it has to be highly compressed (& is highly volatile).