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Ecologists have developed terminology to distinguish ecosystems based on the degree of human alteration. To this end, ecosystems can be characterized as "novel ecosystems," "impacted ecosystems," or "designed ecosystems," depending on the role of human management in ecosystem development and effects on ecosystem properties. Properly classifying an ecosystem as novel, impacted, or designed has critical implications for its conservation and management, but a broadly applicable definition for a "novel ecosystem" does not exist. We have provided a formal definition of "novel ecosystem" that facilitates its use in practical applications and have described four characteristics of such an ecosystem. A novel ecosystem can be identified by its origins rooted in human agency, the ecological thresholds it has crossed, a significantly altered species composition, and a capacity to sustain itself. Ecosystem classification in the literature has been inconsistent. We have illustrated the application of our definition using multiple case studies representing impacted, designed, and novel ecosystems.