Noun
English Wikipedia has an article on:X-bar theoryWikipedia
X-bar theory (uncountable)
(linguistics) A theory that posits a common syntactic structure for different lexical categories. Each phrase (XP) is the projection of a head (X) and may optionally include a specifier (sister of X-bar, daughter of XP), adjuncts (sister and daughter of X-bar) and complements (sister of X, daughter of X-bar).