What is the fundamental characteristic of a hematoma?

The fundamental characteristic of a hematoma is that it contains blood. A hematoma is defined as a localized collection of blood outside of blood vessels, typically resulting from trauma or injury that leads to blood leakage into surrounding tissues. This accumulation can occur in various locations within the body, and the blood can clot, which may lead to swelling and discoloration.

While a hematoma can sometimes be a solid structure due to the clotting of blood, this is not its defining feature. Pain can also be associated with a hematoma, but it is not universally present in all cases. Moreover, hematomas typically occur due to some form of trauma or injury; suggesting that they can form without injury is misleading, as it is this injury that causes the blood to collect outside the vessels. Thus, the presence of blood is the key aspect that characterizes a hematoma, making it the correct answer.