Symmetry is the balanced distribution of duplicate body parts or shapes within the body of an organism.

Symmetry is always approximate: for example plant leaves, while considered symmetrical, rarely match up exactly when folded in half. The body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical. A small minority, notably among the sponges, exhibit no symmetry (i.e., are asymmetric).

source: Wikipedia page on biological symmetry