The Supreme Court in Weyerhaeuser Co. v. United States Fish and Wildlife Service ruled that “critical habitat” under the Endangered Species Act (ESA) must first be “habitat,” but it did not attempt to define exactly what habitat is or how much deference the federal wildlife agencies should get on what is both a biological and policy question. The Court also sidestepped whether currently unoccupied habitat must in fact be “habitable” at the time of designation as critical habitat. The task of defining “habitat” now falls to the ESA’s implementing agencies or to Congress. How habitat is ultimately defined has serious implications for species conservation. In the wake of recent reports on the accelerating loss of biodiversity in the United States and across the globe due largely to habitat loss, how and where we protect habitat is vital to preventing extinction and ensuring the long-term security of species. A definition that is too narrow and excludes degraded but restorable habitat, or areas that are likely to become habitat in the foreseeable future, could leave areas essential to species recovery unprotected. It is, however, possible to define habitat in a way that is consistent with the intent of the ESA, reflects the best available science, is operationally workable, and also broad enough to account for species’ needs. This paper proposes such a definition.