fissure. n. 1. any long narrow cleft or crack, esp in a rock. 2. a weakness or flaw indicating impending disruption or discord: fissures in a decaying empire.3. (Anatomy) anatomy a narrow split or groove that divides an organ such as the brain, lung, or liver into lobes.See also sulcus. 4. (Medicine) a small unnatural crack in the skin or mucous membrane, as between the toes or at the anus. 5.ilum, hilus - (anatomy) a depression or fissure where vessels or nerves or ducts enter a bodily organ; the hilus of the kidney. anatomy, general anatomy-the branch of morphology that deals with the structure of animals.