The Collaborative International Dictionary of English v.0.48:

Zygomorphic \Zyg`o*mor"phic\, Zygomorphous \Zyg`o*mor"phous\, a.
   [Gr. zygo`n a yoke + ? form.] (Biol.)
   Symmetrical bilaterally; -- said of organisms, or parts of
   organisms, capable of division into two symmetrical halves
   only in a single plane.
   [1913 Webster]

WordNet (r) 3.0 (2006):

zygomorphic
    adj 1: capable of division into symmetrical halves by only one
           longitudinal plane passing through the axis [syn:
           zygomorphic, bilaterally symmetrical, zygomorphous]
           [ant: actinomorphic, actinomorphous]