Source : Webster's Revised Unabridged Dictionary (1913)

Cumulus \Cu"mu*lus\ (k?"m?-l?s), n.; pl. {Cumuli} (-l?). [L., a
   heap. See {Cumber}.] (Meteor.)
   One of the four principal forms of clouds. See{Cloud}.

Source : WordNet®

cumuli
     See {cumulus}

cumulus
     n 1: a globular cloud [syn: {cumulus cloud}]
     2: a collection of objects laid on top of each other [syn: {pile},
         {heap}, {mound}]
     [also: {cumuli} (pl)]