Let $G$ be a graph with vertex sex $V(G)$ and let $d(x)$ be the degree of the vertex $x \in V(G)$. The graph invariant $F = \sum\limits_{x \in V(G)} d(x)^3$ played some role in a paper published in 1972, but has not attracted any attention until quite recently. In 2014 an unexpected chemical application of the $F$-index was discovered, which motivated us to establish its basic mathematical properties. Results obtained along these lines are presented.