Final answer:
A bubble of blood under the skin is known as a hematoma, a condition where blood pools beneath the skin's surface due to a severed blood vessel.
Step-by-step explanation:
The term for a bubble of blood under the skin is a hematoma. Hematomas occur when blood from a severed blood vessel pools beneath the skin, resulting in a visible bruise or bubble-like swelling. While terms like petechiae, ecchymosis, and purpura also describe skin discolorations involving bleeding beneath the surface, each represents a different kind and scale of bleeding. Petechiae, for example, are tiny, pinpoint red or purple spots on the skin resulting from leakage out of damaged vessels, which is not the same as the larger collection of blood described in hematomas.