Final answer:
A hematoma is a localized collection of usually clotted blood in an organ, tissue, or space, caused by a break in the wall of a blood vessel.
Step-by-step explanation:
A localized collection of blood, usually clotted, in an organ, tissue, or space, caused by a break in the wall of a blood vessel, is known as a hematoma. This can occur when blood vessels are damaged, allowing blood to seep out and clump together outside of the vascular system. When even a small blood vessel is severed, blood components such as erythrocytes (red blood cells) and leukocytes (white blood cells) escape, leading to the formation of a hematoma. In the case of thrombi, a thrombus is an aggregation of platelets, erythrocytes, white blood cells, and fibrin strands that form within a blood vessel, which can lead to significant health issues if it impedes blood flow or breaks free to become an embolus that causes an embolism.