(1) A two-dimensional diagram used in generative grammar as a convenient means of displaying the internal hierarchical structure of sentences as generated by a set of rules. The ‘root’ of the tree diagram is at the top of the diagram, consisting of the initial symbol S. From this topmost point, or node, branches descend corresponding to the categories specified by the rules (e.g. NP, VP). The internal relationships of parts of the tree are described using ‘family tree’ terminology: if two categories both derive from a single node, they are said to be ‘sisters’, and ‘daughters’ of the ‘mother node’ from which they derive. A subsection of a tree diagram, isolated for purposes of discussion, is referred to as a subtree, as in the enclosed area within the diagram below. The internal organization of a tree is sometimes referred to as tree geometry. In generalized phrase-structure grammar, the term local tree refers to a tree of depth one, i.e. a tree in which every node other than the root is a daughter of the root. The S-NP-VP subtree in the diagram below would be a local tree, in this context. In procedural grammar, a structure tree or parse tree is the result of applying the analytical procedures to a text. In computer corpus research, a parsed corpus is known as a treebank.

(2) In historical linguistics, a representation of the genetic relationships between the members of a family of languages.