The term multigraph refers to a graph in which multiple edges between nodes are either permitted (Harary 1994, p. 10; Gross and
Yellen 1999, p. 4) or required (Skiena 1990, p. 89, Pemmaraju and Skiena
2003, p. 198; Zwillinger 2003, p. 220). West (2000, p. xiv) recommends
avoiding the term altogether on the grounds of this ambiguity.
Some references require that multigraphs possess no graph loops (Harary 1994, p. 10; Gross and Yellen 1999, p. 4; Zwillinger
2003, p. 220), some explicitly allow them (Hartsfield and Ringel 1994, p. 7;
Cormen et al. 2001, p. 89), and yet others do not include any explicit
allowance or disallowance (Skiena 1990, p. 89; Gross and Yellen 1999, p. 351;
Pemmaraju and Skiena 2003, p. 198). Worse still, Tutte (1998, p. 2) uses
the term "multigraph" to mean a graph containing either loops or
multiple edges.
As a result of these many ambiguities, use of the term "multigraph" should be deprecated, or at the very least used with extreme caution.
