From AMS Glossary
A species of cumulonimbus cloud characterized by the presence, mostly in its upper portion, of distinct cirriform parts, frequently in the form of an anvil (incus), a plume, or a vast and more or less disorderly mass of hair.
This cloud is usually accompanied by a shower or thunderstorm, often by a squall, and sometimes by hail. It generally produces very apparent virga. The species capillatus is unique to the genus cumulonimbus.