Edit Mode – mesh – Symmetry Tool
The Symmetrize tool is a quick way to make a mesh symmetrical. Symmetrize works by cutting the mesh at the pivot point of the object, and mirroring over the geometry in the specified axis, and merges the two halves together (if they are connected). Also the mesh data is copied from one side to the other: e.g. UVs, vertex colors, vertex weights.
Examples