symmetry

the quality of having parts that match each other, especially in a way that is attractive, or similarity of shape or contents:

Examples:

in mathematics, the quality of having two parts that match exactly, either when one half is like an image of the other half in a mirror, or when one part can take the place of another if it is turned 90° or 180°

the quality of having parts on either side or half that match each other, esp. in an attractive way, or are the same size or shape