The chapter analyses the uses made in the economic literature of the two modern names of the Paretian criterion: optimality and efficiency. Based on a large database of 10,000 EconLit referenced scientific publications, we use a lexicometric and historical approach to study the uses, instead of the meaning, of the Paretian Criterion. We prove that economists differentiate their use of optimality and efficiency and that since the 1970s, uses of Pareto-efficiency have gradually replaced those of Pareto-optimality. Our interpretation is that economists who use the Paretian criterion change the status of their analysis from normative to positive economics.