Definition of "cumulus" [cu•mu•lus]

• A dense, white, fluffy, flat-based cloud with a multiple rounded top and a well-defined outline, usually formed by the ascent of thermally unstable air masses. (noun)
• A pile, mound, or heap. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The word cumulus has a root similar to accumulate and means “piled up.”"
• "The first stage of air mass thunderstorm development is called the cumulus stage (Figure 3)."
• "This works to loosen a gel-like coating and sticky layer of cells known as the cumulus corona."