A hematoma, or haematoma, is a localized collection of blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 outside the blood vessels, usually in liquid form within the tissue. This distinguishes it from an ecchymosis

Ecchymosis

An ecchymosis is the medical term for a subcutaneous purpura larger than 1 centimeter or a hematoma, commonly called a bruise. It can be located in the skin or in a mucous membrane.-Presentation:...

, which is the spread of blood under the skin in a thin layer, commonly called a bruise

Bruise

A bruise, also called a contusion, is a type of relatively minor hematoma of tissue in which capillaries and sometimes venules are damaged by trauma, allowing blood to seep into the surrounding interstitial tissues. Bruises can involve capillaries at the level of skin, subcutaneous tissue, muscle,...

. Internal bleeding is generally considered to be a spreading of blood within the abdomen or skull, not within muscle.

It is not to be confused with hemangioma

Hemangioma

A hemangioma of infancy is a benign self-involuting tumor of endothelial cells, the cells that line blood vessels. It usually appears during the first weeks of life and sometimes resolves by age 10. In more severe case hemangioma may have permanency, if not treated by a physician...

 which is an abnormal build up of blood vessels in the skin or internal organs.
The word "hematoma" came into usage around 1850.