Editing Tree structure (section)

==Terminology and properties==
The tree elements are called "[[Node (computer science)|node]]s".
The lines connecting elements are called "branches". 
Nodes without children are called [[leaf node]]s, "end-nodes", or "leaves".

Every [[Finite set|finite]] tree structure has a member that has no [[superior (hierarchy)|superior]]. This member is called the "root" or [[root node]]. The root is the starting node. But the converse is not true: infinite tree structures may or may not have a root node.

The names of relationships between nodes model the [[kinship terminology]] of family relations. The gender-neutral names "parent" and "child" have largely displaced the older "father" and "son" terminology. The term "uncle" is still widely used for other nodes at the same level as the parent, although it is sometimes replaced with gender-neutral terms like "ommer".<ref>{{cite web |title=Ethereum Glossary |url=https://github.com/ethereum/wiki/wiki/Glossary |website=GitHub |access-date=17 April 2019 |archive-date=25 April 2019 |archive-url=https://web.archive.org/web/20190425135357/https://github.com/ethereum/wiki/wiki/Glossary |url-status=live }}</ref>

* A node's "parent" is a node one step higher in the hierarchy (i.e. closer to the root node) and lying on the same branch.
* "Sibling" ("brother" or "sister") nodes share the same parent node.
* A node's "uncles" (sometimes "ommers") are siblings of that node's parent.
* A node that is connected to all lower-level nodes is called an "ancestor".  The connected lower-level nodes are "descendants" of the ancestor node.

In the example, "encyclopedia" is the parent of "science" and "culture", its children.  "Art" and "craft" are siblings, and children of "culture", which is their parent and thus one of their ancestors.  Also, "encyclopedia", as the root of the tree, is the ancestor of "science", "culture", "art" and "craft".  Finally, "science", "art" and "craft", as leaves, are ancestors of no other node.

Tree structures can depict all kinds of [[Taxonomy (general)|taxonomic]] knowledge, such as [[family tree]]s, the biological [[evolutionary tree]], the [[Indo-European languages#Classification|evolutionary tree of a language family]], the [[Generative grammar#Context-free grammars|grammatical structure]] of a language (a key example being S → NP VP, meaning a sentence is a noun phrase and a verb phrase, with each in turn having other components which have other components), the way web pages are logically ordered in a web site, [[Tree of primitive Pythagorean triples|mathematical trees of integer sets]], et cetera.

The [[Oxford English Dictionary]] records use of both the terms "tree structure" and "tree-diagram" from 1965 in [[Noam Chomsky]]'s ''[[Aspects of the Theory of Syntax]]''.<ref>
{{OED | tree}}
</ref>

In a tree structure there is one and only one [[path (graph theory)|path]] from any point to any other point.

[[Computer science]] uses tree structures extensively (''see'' [[Tree (data structure)]] and [[telecommunications]].)

For a formal definition see [[Tree (set theory)|set theory]], and for a generalization in which children are not necessarily successors, see [[prefix order]].