脂肪肝（英语：fatty liver disease，简写FLD），又叫肝积脂病或肝性肥胖症，是可逆转的病情。大甘油三酸酯脂肪液胞通过脂肪变性（steatosis）积累在肝细胞（Hepatocyte）。这是用鹅制作鹅肝（法文：Foie gras）的理想状况。虽然有不同原因，但一般认为脂肪肝在全世界范围内都是发生在酗酒（即酒精性脂肪肝，常称酒精肝）和肥胖（不管有没有胰岛素抵抗效应）人群的单一疾病。脂肪肝还与其他影响脂肪代谢的疾病相关。从形态上很难分辨酒精脂肪肝和非酒精脂肪肝，它们都在不同阶段显示微囊泡和巨囊泡脂肪改变。

Fatty liver, also known as fatty liver disease (FLD), is a reversible condition wherein large vacuoles of triglyceride fat accumulate in liver cells via the process of steatosis (i.e., abnormal retention of lipids within a cell). Despite having multiple causes, fatty liver can be considered a single disease that occurs worldwide in those with excessive alcohol intake and the obese (with or without effects of insulin resistance). The condition is also associated with other diseases that influence fat metabolism. When this process of fat metabolism is disrupted, the fat can accumulate in the liver in excessive amounts, thus resulting in a fatty liver. It is difficult to distinguish alcoholic FLD from nonalcoholic FLD, and both show microvesicular and macrovesicular fatty changes at different stages.