bilateral symmetry 
noun [mass noun]
The property of being divisible into symmetrical halves on either side of a unique plane.With few exceptions, animal color patterns show a high degree of bilateral symmetry in shape of pattern elements....
- Ours, however, is closed along the plane of bilateral symmetry, resembling the two closed valves of a bivalve rather than being flattened.
- Exhibiting external bilateral symmetry about a vertical midline, the human body consists of two enantiomorphs - the right and left sides.