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