What is the exact definition of undirected graph, directed graph, unidirectional graph, bidirectional graph?
I've only ever heard of directed and undirected graphs, whose definitions can be found in Wikipedia. Rudimentary search reveals the following interpretations of unidirectional and bidirectional graphs; it is possible that these terms also have different interpretations:


A unidirectional graph is another name for an undirected graph.
A bidirectional graph is a directed graph obtained from an undirected graph by replacing each edge $\{x,y\}$ by a pair of edges $(x,y),(y,x)$.