Definition: transient random walk What exactly does a "transient random walk on a graph/binary tree" mean? Does it mean that we never return to the origin (assuming there is one as for the tree) or just any vertex of the graph or tree? Thanks.

I not so familiar with the terminology exactly for random walks on graphs, but since any random walk on graph is a Markov Chain, we just can refer to the Wikipedia article about Markov Chains: the state is called transient if there is non-zero probability to never return to that state. The Markov Chain is called transient if any state of it is transient.
Following your logic for the binary tree that would impossible to define transience of a random walk on any digraph in the natural way, because there we may not have an origin.
