Property rights are fundamentals to economic analysis. There is, however, no consensus in the economic literature about what property rights are. Economists define them variously and inconsistently, sometimes in ways that deviate from the conventional understandings of legal scholars and judges. This article explores ways in which definitions of property rights in the economic literature diverge from conventional legal understandings, and how those divergences can create interdisciplinary confusion and bias economic analyses. Indeed, some economists' idiosyncratic definitions of property rights, if used to guide policy, could lead to suboptimal economic outcomes.