Recall that an undirected graph$G$ consists of a set $V$ of vertices and a set $E$ of unordered pairs of vertices. A path in $G$ is a finite sequence of vertices $x_0, \ldots, x_n$ such that each pair $x_i x_{i+1}$ belongs to $E$. If $x_n = x_0$, then the path is called a cycle. A graph is connected if it is nonempty and if for every distinct pair of vertices $x, y$, there is a path for which $x_0 = x$ and $x_n = y$. A graph is acyclic if the only cycles are paths

$x_0, \ldots, x_{n-1}, x_n, x_{n-1}, \ldots, x_0$

where steps are retraced; an acyclic graph is also called a forest. A tree is a connected forest.

Each forest is a disjoint sum (a coproduct in the category of graphs) of trees. Removal of an edge of a tree results in a forest.