  The use of the seniority quantum number in many-body systems is reviewed. A
brief summary is given of its introduction by Racah in the context of atomic
spectroscopy. Several extensions of Racah's original idea are discussed:
seniority for identical nucleons in a single-$j$ shell, its extension to the
case of many, non-degenerate $j$ shells and to systems with neutrons and
protons. To illustrate its usefulness to this day, a recent application of
seniority is presented in Bose--Einstein condensates of atoms with spin.
