> where each node has a single parent except for a specific root element which has none. Slight nitpick: in mathematics, a tree need not have a root. https://

> where each node has a single parent except for a specific root element which has none. Slight nitpick: in mathematics, a tree need not have a root. https://mathworld.wolfram.com/FreeTree.html https://mathworld.wolfram.com/FreeTree.html: “a normal tree with no node singled out for special treatment” https://mathworld.wolfram.com/RootedTree.html https://mathworld.wolfram.com/RootedTree.html: “A rooted tree is a tree in which a special ("labeled") node is singled out. This node is called the "root" or (less commonly) "eve" of the tree. Rooted trees are equivalent to oriented trees (Knuth 1997, pp. 385-399). A tree which is not rooted is sometimes called a free tree, although the unqualified term "tree" generally refers to a free tree.”