Melaena

Definition

Blood in the faeces in which dark, tarry masses are passed from the bowel. It is due to bleeding from the stomach or from the higher part of the bowel, the blood undergoing chemical changes under the action of the secretions, and being finally converted in large part into sulphide of iron. It is a serious symptom meriting medical investigation. It can indicate peptic ulcer or carcinoma of the stomach.

Source: Black's Medical Dictionary (42nd ed, 2010, ISBN 978-1-4081-0419-4)

Compare with haematochezia.