Medical Editor: Melissa Conrad Stöppler, MD What is a hematoma? Hematoma is generally defined as a collection of blood outside of blood vessels. Most commonly, hematomas are caused by an injury to the wall of a blood vessel, prompting blood to seep out of the blood vessel into the surrounding tissues. A hematoma can result from an injury to any type of blood vessel (artery, vein, or small capillary). A hematoma usually describes bleeding which has more or less clotted, whereas a hemorrhage signifies active, ongoing bleeding. Hematoma is a very common problem encountered by many people at some time in their lives. Hematomas can be seen under the skin or nails as purplish bruises of different sizes. Skin bruises can also be called contusions. Hematomas can also happen deep inside the body where they may not be visible. Hematomas may sometimes form a mass or lump that can be felt. Sometimes hematomas are named based on their location. Some examples include: Subdural hematoma: a hematoma between the brain tissue and the inside lining of the brainSpinal epidural hematoma: a hematoma between spinal vertebrae and the outside lining of the spinal cordIntracranial epidural hematoma: a hematoma between the skull and the outside lining of the brainSubungual hematoma: a hematoma under the nailIntra-abdominal, peritoneal, or retroperitoneal hematoma: a hematoma inside the abdominal cavityEar or aural hematoma: a hematoma between the ear cartilage and overlying skinSplenic hematoma: a hematoma within the spleenHepatic hematoma: a hematoma within the liver Most hematomas resolve spontaneously over time as the blood debris is removed and the blood vessel wall is repaired by the body's repair mechanisms. Other times, surgically removing or evacuating the blood in a hematoma becomes