The Collaborative International Dictionary of English v.0.48:

Cumulus \Cu"mu*lus\ (k[=u]"m[-u]*l[u^]s), n.; pl. Cumuli
   (-l[imac]). [L., a heap. See Cumber.] (Meteor.)
   One of the four principal forms of clouds. SeeCloud.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cloud \Cloud\ (kloud), n. [Prob. fr. AS. cl[=u]d a rock or
   hillock, the application arising from the frequent
   resemblance of clouds to rocks or hillocks in the sky or
   air.]
   1. A collection of visible vapor, or watery particles,
      suspended in the upper atmosphere.
      [1913 Webster]

            I do set my bow in the cloud.         --Gen. ix. 13.
      [1913 Webster]

   Note: A classification of clouds according to their chief
         forms was first proposed by the meteorologist Howard,
         and this is still substantially employed. The following
         varieties and subvarieties are recognized:
      (a) Cirrus. This is the most elevated of all the forms
          of clouds; is thin, long-drawn, sometimes looking like
          carded wool or hair, sometimes like a brush or room,
          sometimes in curl-like or fleecelike patches. It is
          the cat's-tail of the sailor, and the mare's-tail of
          the landsman.
      (b) Cumulus. This form appears in large masses of a
          hemispherical form, or nearly so, above, but flat
          below, one often piled above another, forming great
          clouds, common in the summer, and presenting the
          appearance of gigantic mountains crowned with snow. It
          often affords rain and thunder gusts.
      (c) Stratus. This form appears in layers or bands
          extending horizontally.
      (d) Nimbus. This form is characterized by its uniform
          gray tint and ragged edges; it covers the sky in
          seasons of continued rain, as in easterly storms, and
          is the proper rain cloud. The name is sometimes used
          to denote a raining cumulus, or cumulostratus.
      (e) Cirro-cumulus. This form consists, like the cirrus,
          of thin, broken, fleecelice clouds, but the parts are
          more or less rounded and regulary grouped. It is
          popularly called mackerel sky.
      (f) Cirro-stratus. In this form the patches of cirrus
          coalesce in long strata, between cirrus and stratus.
      (g) Cumulo-stratus. A form between cumulus and stratus,
          often assuming at the horizon a black or bluish tint.
          -- Fog, cloud, motionless, or nearly so, lying near
          or in contact with the earth's surface. -- Storm
          scud, cloud lying quite low, without form, and driven
          rapidly with the wind.
          [1913 Webster]

   2. A mass or volume of smoke, or flying dust, resembling
      vapor. "A thick cloud of incense." --Ezek. viii. 11.
      [1913 Webster]

   3. A dark vein or spot on a lighter material, as in marble;
      hence, a blemish or defect; as, a cloud upon one's
      reputation; a cloud on a title.
      [1913 Webster]

   4. That which has a dark, lowering, or threatening aspect;
      that which temporarily overshadows, obscures, or
      depresses; as, a cloud of sorrow; a cloud of war; a cloud
      upon the intellect.
      [1913 Webster]

   5. A great crowd or multitude; a vast collection. "So great a
      cloud of witnesses." --Heb. xii. 1.
      [1913 Webster]

   6. A large, loosely-knitted scarf, worn by women about the
      head.
      [1913 Webster]

   Cloud on a (or the) title (Law), a defect of title,
      usually superficial and capable of removal by release,
      decision in equity, or legislation.

   To be under a cloud, to be under suspicion or in disgrace;
      to be in disfavor.

   In the clouds, in the realm of facy and imagination; beyond
      reason; visionary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cumulus
    n 1: a globular cloud [syn: cumulus, cumulus cloud]
    2: a collection of objects laid on top of each other [syn:
       pile, heap, mound, agglomerate, cumulation,
       cumulus]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "cumulus":
   Cloudcuckooland, alto-cumulus, alto-stratus, anvil cloud,
   banner cloud, billowy cloud, cap cloud, cirro-cumulus,
   cirro-fillum, cirro-nebula, cirro-stratus, cirro-velum, cirrus,
   cirrus cloud, cirrus haze, cirrus stripe, cloud, cloud band,
   cloud bank, cloud drift, cloud mass, cloud street, cloudland,
   cloudling, cloudscape, cocktail, cottony cloud,
   cumulo-cirro-stratus, cumulo-cirrus, cumulo-nimbus, cumulo-stratus,
   cumulus cloud, curl cloud, fleecy cloud, fractocumulus, high fog,
   mackerel sky, mammatocumulus, mushroom cloud, nimbus, nimbus cloud,
   rain cloud, scud, snail cloud, squall cloud, storm cloud,
   strato-cumulus, stratus, stratus cloud, thundercloud, thunderhead,
   water carrier, woolpack