A syntactic theory that postulates intermediate constituents below the phrase level (XP) but above the word level (X), known as "X-bar" (written "X̅" or "X′").
The X-bar theory is a theory of syntax that postulates X-bar (written "X̅" or "X′") categories of phrases, where "X" can be a lexical-category such as N (noun). The X̅ category shows up between X and XP in a parse-tree. An X̅ consists of an X possibly combined with an adjunct or with complements. An XP consists of an X̅ possibly with a specifier.