A fissure is a narrow, deep crack or split in something. It can refer to a break or division in a surface, such as a rock or piece of pottery, where two sides have separated along a plane. In medicine, it refers to a small tear or opening in the skin or mucous membrane, often caused by strain or injury. Fissure can also be used metaphorically to describe a division or gap between people or groups, or a disagreement within an organization.