Disease heterogeneity is the phenomenon characterized by significant variations in clinical presentation, underlying biological mechanisms, pathological features, progression patterns, and responses to treatment among individuals diagnosed with the same disease entity. As a fundamental research concept, it involves the systematic investigation and classification of these diverse manifestations and etiologies to identify distinct subtypes, endotypes, or subgroups within a broadly defined disease category. Understanding disease heterogeneity is crucial for advancing precision medicine, improving diagnostic accuracy, predicting prognosis, and developing targeted therapeutic strategies optimized for specific patient populations or even individual patients.