The Collaborative International Dictionary of English v.0.48:

Zoon \Zo"on\, n.; pl. Zoa. [NL., fr. Gr. zw^,on an animal.]
   (Zool.)
      (a) An animal which is the sole product of a single egg;
          -- opposed to zooid. --H. Spencer.
      (b) Any one of the perfectly developed individuals of a
          compound animal.
          [1913 Webster]