medical definition of fissure
fissure. 1. Anatomy a narrow split or groove that divides an organ such as the brain, lung, or liver into lobes. 2. a small unnatural crack in the skin or mucous membrane, as between the toes or at the anus. 3. a minute crack in the surface of a tooth, caused by imperfect joining of enamel during development. [ â²fishÂ·Ér ].