The Collaborative International Dictionary of English v.0.48:

Agenesis \A*gen"e*sis\, n. [Gr. 'a priv. + ? birth.] (Physiol.)
   Any imperfect development of the body, or any anomaly of
   organization.
   [1913 Webster]

WordNet (r) 3.0 (2006):

agenesis
    n 1: imperfect development; nondevelopment of a part [syn:
         agenesis, agenesia]