multigraph

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Multigraph

A multigraph with a loop (blue) and three edges with the same endpoints (all red). Not all authors allow multigraphs to have loops.

From multi- +‎ graph. Compare multiset.

multigraph (plural multigraphs)

(mathematics, graph theory) A set V (whose elements are called vertices or nodes), taken together with a multiset E, each of whose elements (called an edge or line) is a cardinality-two multisubset of V.
(mathematics, graph theory, less commonly) A set V (as before), taken together with a multiset E, each of whose elements is a cardinality-two subset of V.