Hematoma: A localized swelling that is filled with blood caused by a break in the wall of a blood vessel. The breakage may be spontaneous, as in the case of an aneurysm, or caused by trauma. The blood is usually clotted or partially clotted, and it exists within an organ or in a soft tissue space, such as muscle. Treatment depends on the location and size of the hematoma but usually involves draining the accumulated blood. A hematoma in or near the brain is particularly dangerous.