Symmetry operation
Symmetry is a characteristic of geometrical shapes, equations, and other objects; we say that such an object is symmetric with respect to a given operation if this operation, when applied to the object, does not appear to change it. Two objects are symmetric to each other with respect to a given group of operations if one is obtained from the other by one of the operations. In 2D geometry the main kinds of symmetry of interest are: translations, rotations, reflections, and glide reflections.

To read more:
1. Teaching Pamflet by IUCR
2. Basic Symmetry Operation by Artlandia