One of the four humours of ancient and mediaeval physiology, that was believed to be secreted by the kidneys and spleen and to cause melancholy and sadness when present in excess.
2011, (w), The Emperor of all Maladies, Fourth Estate 2011, p. 49:
The problem with treating cancer surgically, Galen suggested, was that black bile was everywhere, as inevitable and pervasive as any fluid.