Measures of azimuthal anisotropy in high-energy collisions

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.