A tree corresponds to a graph with the root node matching a vertex connected by (directed) arcs to the vertices, which match the root nodes of each of its subtrees. An alternative definition of a (directed) tree can thus be given in terms from graph theory: a tree is a directed acyclic graph such that firstly there is a unique vertex, which no arcs enter, called the root, secondly every other vertex has exactly one arc entering it, and thirdly there is a unique path from the root to any vertex.
The diagram shows different representations of a tree.
The terminology associated with trees is either of a botanic nature, as with forest, leaf, root, or is genealogical, as with ancestor, descendant, child, parent, sibling. See also binary tree.
Tree.Sample tree represented as a Venn diagram (top) and as a directed graph