The keyword phrase "September 11th Wikipedia" functions as a noun phrase. The main point or grammatical head of this phrase is the proper noun "Wikipedia."
In this construction, "Wikipedia" is a proper noun, referring to the specific online encyclopedia. The term "September 11th" acts as a proper adjective or a noun adjunct, modifying "Wikipedia." It specifies the topic of the particular article being referenced. Therefore, the phrase is not referring to the encyclopedia in general, but to a distinct article within it. The entire unit operates syntactically as a single noun, capable of functioning as the subject or object in a sentence (e.g., "The September 11th Wikipedia article is extensive.").
Understanding that the core element is a noun is crucial for semantic analysis and information retrieval. It establishes the primary subject of the query as a specific document ("the article") rather than just a date or a general concept. The adjectival modifier provides the necessary context to identify which specific document is intended.
