The Collaborative International Dictionary of English v.0.48:

Aplasia \A*pla"si*a\, n. [NL.; Gr. ? priv. + ? a molding.]
   (Med.)
   1. Incomplete or faulty development.
      [Webster 1913 Suppl.]

   2. (Med.) a defect, retardation, or cessation of the process
      of generation of blood cells.
      [PJC]

WordNet (r) 3.0 (2006):

aplasia
    n 1: failure of some tissue or organ to develop