A hematoma is a localized collection of blood outside of the blood vessels, usually caused by trauma or injury. It can occur in various parts of the body, including the skin, muscles, organs, and brain. Hematomas can range in size from small to large and can be painful and potentially dangerous if they compress nearby structures or become infected. Treatment for a hematoma typically involves rest, ice, compression, and elevation, and in some cases, drainage or surgical intervention may be necessary.