Learn in the textbookBasics of Graph Theory
DefinitionAn undirected graph (無向グラフ) is a graph whose edges have no direction and are drawn as lines without arrowheads.
ExplanationThe two vertices joined by an edge are equal partners, and the relationship is the same from either side. Undirected graphs represent things like people who know each other and road networks you can travel in both directions.
AnalogyThink of a handshake. If A is shaking hands with B, then B is shaking hands with A.
Memory hookUndirected is directed with un- in front. The un- tells you there is no arrowhead.