The use of biomass as an energy feedstock is emerging as a potentially viable alternative to address U.S. energy security concerns, foreign oil dependence, rural economic development, and diminishing sources of conventional energy. Biomass (organic matter that can be converted into energy) may include food crops, crops for energy (e.g., switchgrass or prairie perennials), crop residues, wood waste and byproducts, and animal manure. Most legislation involving biomass has focused on encouraging the production of liquid fuels from corn. Efforts to promote the use of biomass for power generation have focused on wood, wood residues, and milling waste. Comparatively less emphasis has been placed on the use of non-corn-based biomass feedstocks-other food crops, non-food crops, crop residues, animal manure, and more-as renewable energy sources for liquid fuel use or for power generation. This is partly due to the variety, lack of availability, and dispersed location of non-corn-based biomass feedstock. The technology development status and costs to convert non-corn-based biomass into energy are also viewed by some as an obstacle to rapid technology deployment. For over 30 years, the term biomass has been a part of legislation enacted by Congress for various programs, indicating some interest by the general public and policymakers in expanding its use. To aid understanding of why U.S. consumers, utility groups, refinery managers, and others have not fully adopted biomass as an energy resource, this report investigates the characterization of biomass in legislation. The definition of biomass has evolved over time, most notably since 2004. The report lists biomass definitions enacted by Congress in legislation and the tax code since 2004 and definitions contained in legislation from the 111th Congress (the American Clean Energy and Security Act of 2009, H.R. 2454; the American Clean Energy Leadership Act of 2009, S. 1462; the Clean Energy Jobs and American Power Act, S. 1733; and the discussion draft of the American Power Act). Comments on the similarities and differences among the definitions are provided. One point of contention regarding the definition is the inclusion of biomass from federal lands. Some argue that removal of biomass from these lands may lead to ecological harm. Others contend that biomass from federal lands can aid the production of renewable energy to meet certain mandates (e.g., the Renewable Fuel Standard) and that removal of biomass can enhance forest protection from wildfires. Factors that may prevent a private landowner from rapidly entering the biomass feedstock market are also included in the report. Bills were introduced in the 112th Congress that would modify the biomass definition (e.g., S. 781, H.R. 1861). However, debates about the definition have not been as extensive in the 112th Congress as they were in previous Congresses. Forthcoming discussions about energy, particularly legislation involving the Renewable Fuel Standard or energy tax incentives, may prompt further discussion about the definition of biomass.