specifier (n.) (spec, Spec)

A relation in the X-BAR theory of PHRASE STRUCTURE. Specifiers are normally seen as combining with a SINGLE-BAR category to form the related DOUBLE-BAR category. For example, in John is a student, a is the specifier of the NOUN, student, and in She is very happy, she is the specifier of the ADJECTIVE, happy. Spec/head agreement is agreement between the head of a phrase and the element which occupies the specifier of that phrase.