Specific energy
The specific energy of a compound is the energy content per mass unit. The SI-unit for specific energy is joules per kilogram, J/kg.
A closely related measure is the energy density of a compound, which is the energy content per volume unit. The SI-unit for energy density is joules per cubic meters, J/m3.
Gaseous fuels, like hydrogen gas or natural gas, have very high specific energies, but very low energy densities because the gases take up a large volume. Energy density is a particularly important property of a fuel if it is to be used for transportation purposes, where the size of the fuel tank is limited.