Is 2-multiset a valid term? I am trying to describe the edges of an undirected graph that contains loops. On Wikipedia they are characterized as 2-multisets, meaning it has two elements which can be identical, and the order is not important. I never heard of that term, and find no reference to it anywhere else, so I am not sure whether it is OK to use it in my thesis. Can anyone clear that up for me?

Yes. See the Wikipedia article on multisets and the many references therein for where it has occurred previously in literature. 
(Note, in the context, a 2-multiset merely means a multiset containing 2 elements.)
