Internal bleeding: Bleeding inside the body that is not seen from the outside. Internal bleeding occurs when damage to an artery or vein allows blood to escape the circulatory system and collect inside the body. The internal bleeding may occur within tissues, organs, or in cavities of the body including the head, chest, and abdomen. Examples of other potential sites of bleeding include the eye, lining tissues of the heart, muscles, and joints. Internal bleeding may not be evident for many hours after it begins, and symptoms occur when there is significant blood loss or if a blood clot is large enough to compress an organ and prevent it from functioning properly.