The Berlin Definition of Acute Respiratory Distress Syndrome (ARDS) was developed to improve the accuracy and reliability of diagnosing ARDS. The definition incorporates a conceptual model that emphasizes feasibility, reliability, and validity. It includes criteria such as the timing of onset, bilateral opacities on chest imaging, and hypoxemia, with specific thresholds for the PaO₂/FiO₂ ratio to categorize ARDS into mild, moderate, and severe levels. The definition also removes the pulmonary artery wedge pressure criterion for hydrostatic edema and includes illustrative vignettes to guide the assessment of the primary cause of respiratory failure. The Berlin Definition aims to better categorize patients based on severity and improve treatment outcomes by aligning therapeutic options with the severity of the condition. It also addresses limitations of the previous American-European Consensus Conference (AECC) definition, such as poor inter-observer reliability in chest X-ray interpretation and the inclusion of hydrostatic edema as a potential alternative diagnosis. The new definition is supported by clinical trials and is endorsed by major medical societies. The Berlin Definition is expected to facilitate case recognition and improve the matching of treatment options to the severity of illness in both research and clinical practice. The definition is based on a consensus process involving experts from various fields and includes supplementary material to aid in its application. The definition is subject to ongoing evaluation and refinement as new data and experience become available.The Berlin Definition of Acute Respiratory Distress Syndrome (ARDS) was developed to improve the accuracy and reliability of diagnosing ARDS. The definition incorporates a conceptual model that emphasizes feasibility, reliability, and validity. It includes criteria such as the timing of onset, bilateral opacities on chest imaging, and hypoxemia, with specific thresholds for the PaO₂/FiO₂ ratio to categorize ARDS into mild, moderate, and severe levels. The definition also removes the pulmonary artery wedge pressure criterion for hydrostatic edema and includes illustrative vignettes to guide the assessment of the primary cause of respiratory failure. The Berlin Definition aims to better categorize patients based on severity and improve treatment outcomes by aligning therapeutic options with the severity of the condition. It also addresses limitations of the previous American-European Consensus Conference (AECC) definition, such as poor inter-observer reliability in chest X-ray interpretation and the inclusion of hydrostatic edema as a potential alternative diagnosis. The new definition is supported by clinical trials and is endorsed by major medical societies. The Berlin Definition is expected to facilitate case recognition and improve the matching of treatment options to the severity of illness in both research and clinical practice. The definition is based on a consensus process involving experts from various fields and includes supplementary material to aid in its application. The definition is subject to ongoing evaluation and refinement as new data and experience become available.