The Collaborative International Dictionary of English v.0.48:

Nucellus \Nu*cel"lus\, n.; pl. Nucelli. [NL., dim. of nux,
   nucis, a nut.] (Bot.)
   See Nucleus, 3
   (a) .
       [1913 Webster]

WordNet (r) 3.0 (2006):

nucellus
    n 1: central part of a plant ovule; contains the embryo sac