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  "body": "\n## Definition\n\nLet $G$ be a [[graph|0JD5C82QMB]] and let $u, v$ be vertices of $G$.\nThe vertex $v$ is reachable from $u$ if there is a [[walk|0VJZM0F9YM]] in $G$ from $u$ to $v$.\n\n## Remarks\n\nReachability is an equivalence relation on the vertices. It is reflexive by the walk of length $0$, symmetric by reversal of a walk, and transitive by concatenation.\nIts equivalence classes are the [[connected components|01VVP10MWJ]] of $G$.\nFor a [[directed graph|0R5HTV7REV]], reachability along arcs is reflexive and transitive, but in general it is not symmetric.\n\n## Explanations\n\n### L1 human\n\nA vertex is reachable from another one when you can go from the first to the second along edges.\nOn a map of islands with bridges, two places are reachable from each other when they are on islands linked by bridges.\n",
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