  Azimuthal anisotropy is a key observation made in ultrarelativistic heavy-ion
collisions. This phenomenon has played a crucial role in the development of the
field over the last two decades. In addition to its interest for studying the
quark-gluon plasma, which was the original motivation, it is sensitive to the
properties of incoming nuclei, in particular to the nuclear deformation and to
the nuclear skin. The azimuthal anisotropy is therefore of crucial importance
when relating low-energy nuclear structure to high-energy nuclear collisions.
This article is an elementary introduction to the various observables used in
order to characterize azimuthal anisotropy, which go under the names of
$v_2\{2\}$, $v_3\{2\}$, $v_2\{4\}$, etc. The intended audience is primarily
physicists working in the field of nuclear structure.
