Biomass is a term used in biology to describe the mass or weight of living organisms in a given area or ecosystem at a given time). It can refer to the mass of one or more species, known as species biomass, or to the mass of all species in a community, known as community biomass). Biomass is important for commercial uses, such as fuel and fiber, and for scientific studies of ecosystem productivity, energy and nutrient flows, and for assessing the contribution of changes in forestlands to the global carbon cycle.

In an ecosystem, biomass is commonly subdivided into five fundamental trophic levels forming the basic food chain: producers, herbivores, predators, top predators, and decomposers. Producers are green photosynthesizing plants that transform sunlight into chemical energy via photosynthesis. Primary consumers are the living organisms that consume producers, while secondary consumers are the organisms that feed on primary consumers.

Biomass can also be used as a source of renewable energy. Organic matter that is used as a source of biomass energy includes trees, corn, rice hulls, peanut shells, sugar cane, grass clippings, leaves, manure, and municipal solid waste. Biomass can be burned directly for heat or converted to liquid and gaseous fuels through various processes.