In a directed graph an (A,B) edge is mutual if the graph also includes a
(B,A) directed edge.
Note that multi-graphs are not handled properly, i.e. if the graph contains
two copies of (A,B) and one copy of (B,A), then these three edges are
considered to be mutual.
Undirected graphs contain only mutual edges by definition.
See Also
reciprocity, dyad_census if you just
want some statistics about mutual edges.