The Supreme Court’s opinion in Weyerhaeuser Co. v. United States Fish and Wildlife Service, 139 S. Ct. 361 (2018), raises important questions about the scope of the Endangered Species Act’s protections for critical habitat. In a short ruling, the Court opined that ‘critical habitat’ must be ‘habitat,’ but it did not attempt to define ‘habitat’ as a scientific or legal matter. Here we provide a brief scientific and legal analysis and propose a definition of ‘habitat’ that the Services could adopt to address this gap.