The "systole" of a compact metric space "X" is a metric invariant of "X", defined to be the least length of a noncontractible loop in "X" (i.e. a loop that cannot be contracted to a point in the ambient space "X"). In more technical language, we minimize length over free loops representing nontrivial conjugacy classes in the fundamental group of "X". When "X" is a graph, the invariant is usually referred to as the girth, ever since the 1947 article on girth by W. T. Tutte. Possibly inspired by Tutte's article, Loewner started thinking about systolic questions on surfaces in the late 1940s, resulting in a 1950 thesis by his student Pao Ming Pu. The actual term "systole" itself was not coined until a quarter century later, by Marcel Berger.