Acute kidney injury (AKI) is a major renal disease that affects 13 million people and is associated with almost 1.7 million deaths each year around the world. Previously referred to as acute renal failure, AKI is defined not only by a rapid functional decline of the kidney but also by pathological damages in kidney tissue. This updated definition contributes to modern renal medicine in at least two ways. First, the definition of AKI includes an early, preventable and treatable stage of the disease, which implies an urgent need for sensitive biomarkers for early diagnosis and treatment. Second, the AKI definition is proposed with clear criteria for diagnosis and severity grading, which are beneficial for clinical studies and the meta‐analysis of different datasets.