Currently, a number of different federal, state, and local definitions of brownfields are in use. I argue here that two types of definitions are required. The first is a broad, universally agreed upon conceptual definition that will permit the creation of brownfield inventories for resource allocation and federal funding purposes and will inform individuals of potential health hazards in their communities. The second type of definition, which assumes the underlying conceptual definition, specifies eligibility qualifications for particular programs. The conceptual definition should contain terms that are unambiguous, and should allow policy makers and practitioners wide latitude in addressing the dual nature of brownfields as both environmental and economic problems. A definition satisfying these criteria is provided in the 2001 Brownfields Revitalization and Environmental Restoration Act. In this article, I trace the evolution of the dual nature of brownfields and offer examples of the various meanings assigned to the term by state and federal entities. I then examine key distinctions made in various definitions and assess their appropriateness as components of a universal conceptual definition. Such a definition must remain true to the environmental and economic development concerns that gave rise to the term. Only the definitional elements central to these concerns should be included — i.e., the presence or potential presence of contamination and the complications this environmental condition presents for redeveloping a site. Further specifications should be relegated to eligibility definitions that designate legal and programmatic qualifications. The analysis indicates that the conceptual definition provided in the 2001 Act is superior to other definitions in use and should be adopted at all levels of government.