The Collaborative International Dictionary of English v.0.48:

zygote \zy"gote\ (z[imac]"g[=o]t), n.
   The cell resulting from the union of an ovum and a
   spermatozoon, or (broadly) the developing new organism
   arising from such a cell.

   Syn: fertilized cell.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

zygote
    n 1: (genetics) the diploid cell resulting from the union of a
         haploid spermatozoon and ovum (including the organism that
         develops from that cell) [syn: zygote, fertilized ovum]