University of California, San Diego: Edward Bender and S. Williamson's "Lists, Decisions, and Graphs: Basic Concepts in Graph Theory"

Read Definition 3 on pages GT-4 to GT-5.

Click https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2011/09/CS202-Graph-Theory-GT-Edward-Bender.pdf link to open resource.