The Collaborative International Dictionary of English v.0.48:

osteoclast \os"te*o*clast\, n. [Osteo- + Gr. kla^n to break.]
   1. (Physiol.) Same as myeloplax, a large multinucleate cell
      within the bone, which functions in the absroption and
      removal of bone tissue.
      [1913 Webster +PJC]

   Note: The osteoclasts occur usually in pits or cavities which
         they appear to have excavated, and are supposed to be
         concerned in the absorption of the bone matrix.
         [1913 Webster]

   2. An instrument for performing osteoclasis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

osteoclast
    n 1: cell that functions in the breakdown and resorption of bone
         tissue