A Symmetry Element is a geometric object that corresponds to a Symmetry Operation.
If the operation is a proper rotation then the symmetry element is the axis of rotation.
If the symmetry operation is a reflection, then the symmetry element is a plane of reflection.
If the symmetry operation is an inversion, then the symmetry element is a point.
If the symmetry operation is an improper rotation then two symmetry elements are present, an axis of rotation and a plane of reflection.
If the operation is identity, we go with the idea that the symmetry element is an arbitrary axis of rotation–any of them will work. E and 
Rob Sterling 