Objectives: The objective was to review and update key definitions and metrics for emergency department (ED) performance and operations.Methods: Forty-five emergency medicine leaders convened for the Third Performance Measures and Benchmarking Summit held in Las Vegas, February 21-22, 2014. Prior to arrival, attendees were assigned to workgroups to review, revise, and update the definitions and vocabulary being used to communicate about ED performance and operations. They were provided with the prior definitions of those consensus summits that were published in 2006 and 2010. Other published definitions from key stakeholders in emergency medicine and health care were also reviewed and circulated. At the summit, key terminology and metrics were discussed and debated. Workgroups communicated online, via teleconference, and finally in a face-to-face meeting to reach consensus regarding their recommendations. Recommendations were then posted and open to a 30-day comment period. Participants then reanalyzed the recommendations, and modifications were made based on consensus.Results: A comprehensive dictionary of ED terminology related to ED performance and operation was developed. This article includes definitions of operating characteristics and internal and external factors relevant to the stratification and categorization of EDs. Time stamps, time intervals, and measures of utilization were defined. Definitions of processes and staffing measures are also presented. Definitions were harmonized with performance measures put forth by the Centers for Medicare and Medicaid Services (CMS) for consistency.Conclusions: Standardized definitions are necessary to improve the comparability of EDs nationally for operations research and practice. More importantly, clear precise definitions describing ED operations are needed for incentive-based pay-for-performance models like those developed by CMS. This document provides a common language for front-line practitioners, managers, health policymakers, and researchers.ACADEMIC EMERGENCY MEDICINE 2015;22:542-553