The Berlin Definition of Acute Respiratory Distress Syndrome (ARDS) is an updated and revised version of the previous American-European Consensus Conference (AECC) definition, developed by a task force of experts from various medical societies. The new definition aims to address the limitations of the AECC definition, such as poor inter-observer reliability in interpreting chest radiographs and the declining use of pulmonary artery catheters. The Berlin Definition maintains key diagnostic criteria, including the onset within one week of a known clinical insult or new/worsening respiratory symptoms, bilateral opacities on chest radiographs, and hypoxemia. However, it introduces several modifications to enhance reliability and validity, such as the removal of the pulmonary artery wedge pressure criterion, the inclusion of a reference set of chest radiographs, and the use of positive end-expiratory pressure (PEEP) criteria to classify severity. The definition also provides illustrative vignettes to guide judgments about the primary cause of respiratory failure and emphasizes the importance of ruling out hydrostatic edema when no risk factor for ARDS is apparent. The Berlin Definition is intended to facilitate case recognition and better match treatment options to severity in both research trials and clinical practice.The Berlin Definition of Acute Respiratory Distress Syndrome (ARDS) is an updated and revised version of the previous American-European Consensus Conference (AECC) definition, developed by a task force of experts from various medical societies. The new definition aims to address the limitations of the AECC definition, such as poor inter-observer reliability in interpreting chest radiographs and the declining use of pulmonary artery catheters. The Berlin Definition maintains key diagnostic criteria, including the onset within one week of a known clinical insult or new/worsening respiratory symptoms, bilateral opacities on chest radiographs, and hypoxemia. However, it introduces several modifications to enhance reliability and validity, such as the removal of the pulmonary artery wedge pressure criterion, the inclusion of a reference set of chest radiographs, and the use of positive end-expiratory pressure (PEEP) criteria to classify severity. The definition also provides illustrative vignettes to guide judgments about the primary cause of respiratory failure and emphasizes the importance of ruling out hydrostatic edema when no risk factor for ARDS is apparent. The Berlin Definition is intended to facilitate case recognition and better match treatment options to severity in both research trials and clinical practice.