The term is a proper noun phrase, specifically a compound noun, used primarily in Spanish-speaking contexts to denote a significant historical event. Grammatically, "11 sep" (an abbreviation for 11 de septiembre) functions as a specifier, identifying the specific date of the event, while "pentagono" (Pentagon) is the noun identifying the location. The entire phrase refers to a singular, specific occurrence, thereby functioning as a proper noun.
This phrase designates the terrorist attack that occurred at the headquarters of the United States Department of Defense in Arlington, Virginia, on September 11, 2001. On that morning, American Airlines Flight 77 was hijacked by five al-Qaeda terrorists and deliberately crashed into the western side of the building at 9:37 AM EDT. The impact and subsequent fire caused a partial collapse of the structure, resulting in the deaths of all 64 people aboard the aircraft and 125 personnel inside the building.
The event was one of four coordinated attacks carried out against the United States on that day, which collectively prompted a fundamental shift in U.S. national security and foreign policy, leading to the initiation of the "War on Terror." The damaged section of the building was reconstructed under a project known as the "Phoenix Project," with its occupants returning to the rebuilt offices by August 2002. An outdoor memorial and an indoor chapel were later constructed on the site to commemorate the victims.
