The Collaborative International Dictionary of English v.0.48:

Rubble \Rub"ble\, n. [From an assumed Old French dim. of robe
   See Rubbish.]
   1. Water-worn or rough broken stones; broken bricks, etc.,
      used in coarse masonry, or to fill up between the facing
      courses of walls.
      [1913 Webster]

            Inside [the wall] there was rubble or mortar.
                                                  --Jowett
                                                  (Thucyd.).
      [1913 Webster]

   2. Rough stone as it comes from the quarry; also, a
      quarryman's term for the upper fragmentary and decomposed
      portion of a mass of stone; brash. --Brande & C.
      [1913 Webster]

   3. (Geol.) A mass or stratum of fragments or rock lying under
      the alluvium, and derived from the neighboring rock.
      --Lyell.
      [1913 Webster]

   4. pl. The whole of the bran of wheat before it is sorted
      into pollard, bran, etc. [Prov. Eng.] --Simmonds.
      [1913 Webster]

   Coursed rubble, rubble masonry in which courses are formed
      by leveling off the work at certain heights.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rubble
    n 1: the remains of something that has been destroyed or broken
         up [syn: debris, dust, junk, rubble, detritus]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "rubble":
   aa, abyssal rock, basalt, bedrock, block lava, brash, breccia,
   clamjamfry, conglomerate, crag, debris, druid stone, dust,
   festooned pahoehoe, gneiss, granite, igneous rock, junk, lava,
   limestone, litter, living rock, lumber, magma, mantlerock,
   metamorphic rock, monolith, pahoehoe, pillow lava, porphyry,
   pudding stone, raff, regolith, riffraff, rock, ropy lava, rubbish,
   rubblestone, sandstone, sarsen, schist, scoria, scrap, scree,
   sedimentary rock, shelly pahoehoe, shoddy, stone, talus, trash,
   truck, tufa, tuff