A hematoma is a collection of blood, usually clotted, outside of a blood vessel that may occur because of an injury to the wall of a blood vessel allowing blood to leak out into tissues where it does not belong. The damaged blood vessel may be an artery, vein, or capillary; the bleeding may be very tiny, with just a dot of blood, or it can be large and cause significant blood loss. It is a type of internal bleeding that is either clotted or is forming clots. Hemorrhage is the term used to describe active bleeding and is often graded on a severity score of one to four (representing 15% to >40% of total blood volume). Hematoma describes bleeding that has already started to become clotted. However, the distinction sometimes is not clear as some hematomas enlarge over time and active bleeding can add to the mass of the hematoma.

Hematomas are often described based upon their location in the body, whether it is in the skull (intracranial), under the fingernail (subungual), or in the earlobe.

Hematomas of the skin may also be named based upon their size. Petechiae are tiny dots of blood usually less than 3 millimeters in diameter (0.12 inch) while purpura are less than 10 millimeters in diameter (0.40 inch) and ecchymosis is greater than 10 millimeters in diameter. Ecchymosis is commonly considered a bruise.

Hematomas form when a blood vessel leaks into surrounding tissue. The injury to a blood vessel wall may occur spontaneously or may be due to injury. The violence of a sneeze or cough may cause blood vessels in the face to break and cause small amounts of bleeding. The body is usually able to repair the damaged vessel wall by activating the blood clotting. Sometimes the repair fails if the damage is extensive and the large defect allows for continued bleeding or if the person has problems with blood clotting. If the bleeding occurs in a tiny capillary blood vessel, only a drop or two of blood may be lost into the surrounding tissue causing petechiae to form. If there is great pressure within the blood vessel, for example a major artery, the blood may continue to leak and cause an expanding hematoma that causes significant blood loss and shock.