n. pl. cu·mu·li (-lī′)
1. 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.
2. A pile, mound, or heap.
[Latin, heap; see keuə- in the Appendix of Indo-European roots.]
(click for a larger image)cumulus
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.