Source : Webster's Revised Unabridged Dictionary (1913)

Zona \Zo"na\, n.; pl. {Zon[ae]}. [L., a girdle. See {Zone}.]
   A zone or band; a layer.

   {Zona pellucida}. [NL.] (Biol.)
   (a) The outer transparent layer, or envelope, of the ovum. It
       is a more or less elastic membrane with radiating
       stri[ae], and corresponds to the cell wall of an ordinary
       cell. See {Ovum}, and Illust. of {Microscope}.
   (b) The zona radiata.

   {Zona radiata} [NL.] (Biol.), a radiately striated membrane
      situated next the yolk of an ovum, or separated from it by
      a very delicate membrane only.

Source : WordNet®

zona pellucida
     n : thick membrane around the mammalian ovum; can be penetrated
         by one sperm in the fertilization process; usually
         remains around the fertilized egg until it is implanted
         in the wall of the uterus