The number of different graphs obtained from the given graph by removing an edge.

The Colin de Verdière graph invariant.

The size of the largest multi-subset-intersection of the deck of a graph with the deck of another graph. The deck of a graph is the multiset of induced subgraphs obtained by deleting a single vertex. The graph reconstruction conjecture states that the deck of a graph with at least three vertices determines the graph. This statistic is only defined for graphs with at least two vertices, because there is only a single graph of the given size otherwise.

The second largest eigenvalue of a graph if it is integral. This statistic is undefined if the second largest eigenvalue of the graph is not integral. Chapter 4 of [1] provides lots of context.

The number of monotone factorisations of genus zero of a permutation of given cycle type. A monotone factorisation of genus zero of a permutation $\pi\in\mathfrak S_n$ with $\ell$ cycles, including fixed points, is a tuple of $r = n - \ell$ transpositions $$ (a_1, b_1),\dots,(a_r, b_r) $$ with $b_1 \leq \dots \leq b_r$ and $a_i < b_i$ for all $i$, whose product, in this order, is $\pi$. For example, the cycle $(2,3,1)$ has the two factorizations $(2,3)(1,3)$ and $(1,2)(2,3)$.