Hematoma: A hematoma is a collection of blood outside of a blood vessel. A hematoma can develop after surgery; this typically results in an area being swollen and bruised in appearance, with a pocket of blood beneath. In some cases, this is minor, but a hematoma can be large enough to cause pain and even decrease blood flow through the area. In the case of a large hematoma, the surgeon may choose to remove some of the collected blood with a syringe or other similar method.