Molecular symmetry
Symmetry axis: an axis around which a rotation by  results in a molecule that appears identical to the molecule before rotation. This is also called an n-fold rotational axis and is shortened to Cn. Examples are the C2 in water and the C3 in ammonia. A molecule can have more than one symmetry axis; the one with the highest n is called the principal axis, and by convention is given the z-axis in a Cartesian coordinate system.