From the U.S. Federal Meteorological Handbook Number 1 - Surface Weather Observations and Reports:
FMH-1
cumulonimbus. An exceptionally dense and vertically developed cloud, occurring either isolated or as a line or wall of clouds with separated upper portions. These clouds appear as mountains or huge towers, at least a part of the upper portions of which are usually smooth, fibrous, or striated, and almost flattened.