An adnominal modifier is an expression that occurs in a nominal phrase and that provides further information beyond the noun.
In the Grammaticon definition, "modifier" is restricted to adnominal use, so this term could simply be "modifier". However, since "modification" is often used more widely in the literature (including adverbials in clauses), the tautological term "adnominal modifier" is used here. ❡
"Attributive" is an older synonym of "adnominal" that is still very widely used.
attributive phrase (CXN) = a construction that performs the act of modification. Example: in a very slow truck, the attributive phrase very slow modifies the truck with respect to its speed. The head of an attributive phrase is a modifier. The prototypical attributive phrase is an adjectival phrase. (Sections 2.2.2, 4.1.1) ❡ modifier (CXN) = the head of an attributive phrase. Example: in nearly fifty trees, fifty is a modifier. A prototypical modifier, a property concept, is an adjective. (Section 2.2.4)