Bile (or gall) is a bitter, greenish-yellow alkaline fluid secreted by the liver of many vertebrates. It is stored in the gallbladder between meals and upon eating is discharged into the duodenum where it aids the process of digestion, especially of fats. Besides its digestive function, bile serves as the route of excretion for hemoglobin breakdown products (bilirubin) which give bile its colour. Bile also contains cholesterol, which occasionally accretes into lumps in the gall bladder, forming gallstones.
Yellow and black bile were two of the four vital fluids or humours of ancient and medieval medicine; for example, melancholia was believed to be caused by a bodily surplus of black bile.