The set 
of edges of a loopless graph 
, being a set of unordered pairs
of elements of 
,
constitutes an adjacency relation on 
. Formally, an adjacency relation is any relation
which is irreflexive and symmetric.
Adjacency Relation
See also
Irreflexive, Relation, SymmetricExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Adjacency Relation." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/AdjacencyRelation.html