Gross hemorrhage into a cavity or into the substance of an organ. The term is now usually limited to cerebral apoplexy, or loss of consciousness due to effusion of blood or other lesion within the substance of the brain causing sudden stroke symptoms, but it is sometimes extended to denote an effusion of blood into the substance of any organ e.g., as apoplexy of the lung.

"It is impossible to cure a severe attack of apoplexy, and difficult to cure a mild one."

– Hippocratic Aphorism

Source: Clarke E. Apoplexy in the hippocratic writings. Bull Hist Med 1963; 37: 301–314.