A graph is "closed" if it contains a cycle that returns to the starting vertex. In other words, you can traverse the graph and end up where you started without retracing edges.
Cycle: A sequence of vertices starting and ending at the same vertex with no repeated edges or vertices except for the starting/ending vertex
Eulerian Circuit: A circuit that visits every edge of a graph exactly once and returns to the starting vertex
Hamiltonian Cycle: A cycle that visits every vertex in a graph exactly once and returns to the starting vertex