Final answer:
A swelling caused by an accumulation of blood under the skin is referred to as a hematoma, which differs from conditions like petechiae and can commonly be confused with edema, a more generalized accumulation of fluid in the tissues.
Step-by-step explanation:
A swelling caused by an accumulation of blood under the skin is known as a hematoma. This condition occurs when blood escapes from the circulatory system and collects in the tissues of the skin or in an organ due to injury or a break in a blood vessel wall. Hematomas can vary in size and may sometimes be confused with other skin conditions, such as petechiae, which are small red or purple spots on the skin that result from blood leaking out of damaged vessels. More serious health issues, like edema, involve the accumulation of excess fluid in the tissues, which can be caused by underlying medical conditions and may lead to swelling and stretched, tight skin. On the other hand, thrombosis is the formation of a blood clot within a blood vessel, which can impede blood flow and potentially lead to serious health risks.