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- n. 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.
Pronunciation of its text representation: a letter X with overbar. (Wiktionary)