A set of circuits going along the Edges of a Graph, each with an Even number of Edges, such that just one of the circuits passes through each Vertex (Ball and Coxeter 1987, pp. 265-266).
See also Edge (Graph), Eulerian Cycle, Hamiltonian Cycle, Tait Coloring, Vertex (Graph)
Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed. New York: Dover, 1987.