Biomass is the animal and plant matter resource that is present in the Biosphere, the thin layer between the heat of the Earth's core and the coldness of space that supports life on this planet.

Biomass from plant matter grows by absorbing water, carbon dioxide, and other nutrients from the air and soil, and is powered through photosynthesis by energy captured from the sun. Further life is sustained via the food chain, with animals which eat plants (herbivores), or other animals (carnivores), or both (omnivores). The energy which is stored in the bodies of these plants and animals, or in the wastes they produce is called Biomass Energy, and is considered a Bio-resource or biomass feedstock. During combustion, biomass fuels release the CO₂ that was collected while the plant was growing.

Biomass is a conventional energy, and people depend on it as a main resource for daily living. In terms of energy capacity, biomass energy ranks just behind coal, oil, and natural gas. Worldwide, biomass is the fourth most consumed energy resource. It plays an important role in the whole energy system.