In generative syntax, a constituent is a notion in the syntactic description of linguistic expressions.
A constituent of an expression is any part of the expression that, linguistically, functions as a unit. In terms of tree structure, a constituent can be defined as a subtree, or as the material dominated by a single node.
In terms of labeled bracketing, the material enclosed by a matching pair of brackets is a constituent.
Each of the bracketed strings in (i) is a constituent:
(i) [1 John [2 left [3 his [4 pet chihuahua 4] 3] [5 to [[6 his mother   2] 1]
Various tests can be employed to establish whether a given part of an expression is a constituent. One well-known test is used in (ii);
a to his mother, John left his pet chihuahua b his pet chihuahua, John left to his mother c * his pet chihuahua to his mother, John left
Assuming that topicalization allows the preposing of exactly one constituent, the well-formedness of (ii)a & b indicates that the italicized strings are constituents; the illformedness of (ii)c can be attributed to the fact that the direct object and the indirect object do not form a constituent. A stricter version of the notion constituent holds that only those subtrees that are maximal projections are called constituents.