from The American Heritage® Dictionary of the English Language, 4th Edition
- n. An extremely dense, vertically developed cumulus with a relatively hazy outline and a glaciated top extending to great heights, usually producing heavy rains, thunderstorms, or hailstorms.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A cloud, with a tall structure and a flat base, that is often associated with thunderstorms.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a dark cloud of great vertical extent charged with electricity; associated with thunderstorms
From Latin cumulus ("amass") + nimbus ("cloud"). (Wiktionary)