a vertex is any vertex that is either the parent of or is (recursively) an ascendant of a parent of . A descendant of a vertex is any vertex that is either a child of or is (recursively) a descendant of a child of . A sibling to a vertex is any other vertex on the tree that shares a parent with . A leaf is a vertex with no children. An internal vertex is a vertex that is not a leaf.

The height of a vertex in a rooted tree is the length of the longest downward path to a leaf from that vertex. The height of the tree is the height of