adjective (a)

• capable of division into symmetrical halves by only one longitudinal plane passing through the axis(adj.all)

  Synonym:
  bilaterally symmetrical, zygomorphous
  Antonym:
  actinomorphic, actinomorphous
  

  source: wordnet30
• Symmetrical bilaterally; -- said of organisms, or parts of organisms, capable of division into two symmetrical halves only in a single plane.(adjective)

  source: webster1913