Specifier

Definition
In the X-bar theory of generative syntax, the specifier is the position which is directly dominated by the maximal projection of X: [XP specifier X].

Many different functions are being assigned to this position, depending on the category of X, such as Determiner of NP, degree element of AP, subject of IP, or modifier (adverb or even auxiliary) of VP. (One version of) the VP-internal Subject Hypothesis holds that Spec,VP is the D-structure position of the verb's external argument. In many analyses of movement (see bounding theory), the specifier position plays an important role as an intermediate landing site (or escape hatch) for A-bar movement.

Links
Utrecht Lexicon of Linguistics