cumulonimbus
Concise Oxford English Dictionary © 2008 Oxford University Press:
cumulonimbus /ˌkjuːmjʊləʊˈnɪmbəs/
▶noun (pl. cumulonimbi /-bʌɪ/) Meteorology cloud forming a towering mass with a flat base at fairly low altitude and often a flat top, as in thunderstorms.
'cumulonimbus' also found in these Oxford entries:

