Definition:Edgeless Graph/Also known as

Edgeless Graph: Also known as
An edgeless graph is sometimes called an empty graph.

Thus the term $n$-empty graph can often be seen for $N_n$.

The symbol $\overline K_n$ is frequently used to denote the $n$-edgeless graph, which follows from Complement of Complete Graph is Edgeless Graph.

The term null graph can also be found, but this can be confused with the graph with no vertices.