X-bar

Pronunciation of its text representation: a letter X with overbar.

X-bar

1. (grammar, X-bar theory) A phrase, or, equivalently, a node in a syntax tree, which consists either of: (1) an adjunct and another X-bar phrase, (2) a head, X, and an optional complement, or (3) a conjunction sandwiched between two other X-bars. The X is a "pro-letter" which can be substituted by letters such as N for noun, V for verb, P for preposition, I for inflectional, etc.
   • 1988, Andrew Radford, chapter 7, in Transformational grammar: a first course, Cambridge, UK: Cambridge University Press, page 350:

     For example, the fact that give must occur as the leftmost constituent of the V-bar containing it follows from two conditions. The first is a putatively universal linearisation (i.e. word-order) principle proposed by Stowell (1981, p. 68) which we might call the PERIPHERY PRINCIPLE: this can be outlined informally as in (33) below:
     (33)      PERIPHERY PRINCIPLE
     (33)      The head term of a Phrase appears at the periphery of X-bar
     What (33) says is that the Head must be the leftmost or rightmost immediate constituent of X-bar.

• An X-bar is denoted as ${\displaystyle {\bar {X}}}$, or more commonly, as ${\displaystyle X'}$.

• I-bar
• N-bar
• P-bar
• V-bar

• adjunct
• head, complement

• XP

• X-bar on Wikipedia.Wikipedia