books.google.com - The systole of a compact metric space $X$ is a metric invariant of $X$, defined as the least length of a noncontractible loop in $X$. When $X$ is a graph, the invariant is usually referred to as the girth, ever since the 1947 article by W. Tutte. The first nontrivial results for systoles of surfaces...http://books.google.com/books/about/Systolic_Geometry_and_Topology.html?id=JuY5tsWAHoUC&utm_source=gb-gplus-shareSystolic Geometry and Topology