What is the medical term meaning abnormal collection of blood under a fingernail or toenail?

A subungual hematoma is blood trapped under a nail. By the next day, it has the characteristic black or very dark blue appearance. It's the familiar "black nail" after whacking your finger with a hammer, for instance. Getting attention within hours of the accident can reduce pain and swelling; after the blood clots, however, the affected person just needs to wait until the swelling goes down naturally.