cu·mu·lus 
(kymyə-ləs)
(kymyə-ləs)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.]
cumulus