noun, plural cumulus.
a heap; pile.
a cloud of a class characterized by dense individual elements in the form of puffs, mounds, or towers, with flat bases and tops that often resemble cauliflower: as such clouds develop vertically, they form cumulonimbus.

1650–60; < Neo-Latin (Latin: mass, pile) Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
cumulus (ˈkjuːmjʊləs)
n , pl -li
1.  cirrus Compare stratus a bulbous or billowing white or dark grey cloud associated with rising air currents
2.  histology the mass of cells surrounding a recently ovulated egg cell in a Graafian follicle
[C17: from Latin: mass]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1650s, "a heap," from L. cumulus "heap," from PIE *ku-m-olo-, suffixed shortened form of base *keue- "to swell" (cf. Skt. svayati "swells up, is strong," Gk. kyein "to swell," Lith. aunas "firm, solid, fit, capable"). Meteorological use for "rounded mass of clouds" first attested 1803.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
cumulus   (kym'yə-ləs)  Pronunciation Key 
Plural cumuli (kym'yə-lī')
A dense, white, fluffy cloud with a flat base, a multiple rounded top, and a well-defined outline. The bases of cumulus clouds form primarily in altitudes below 2,000 m (6,560 ft), but their tops can reach much higher. Cumulus clouds are generally associated with fair weather but can also bring rain when they expand to higher levels. The clouds' edges are well-defined when they are composed of water droplets and fuzzy when made up of ice crystals. See illustration at cloud.
The American Heritage® Science Dictionary
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