The term is a noun phrase, where "9/11" functions as a proper adjective modifying the core noun, "incident." This grammatical structure designates a specific, singular event. The spelling "insiden" is a common phonetic or non-native English rendering of the word "incident," which refers to an event or occurrence.
This phrase specifically refers to the series of four coordinated terrorist attacks by the Islamic militant group al-Qaeda against the United States on the morning of September 11, 2001. The attackers hijacked four commercial airliners. Two planes were flown into the North and South Towers of the World Trade Center complex in New York City, and a third plane hit the Pentagon in Arlington, Virginia. The fourth plane crashed in a field in Shanksville, Pennsylvania, after passengers and crew attempted to regain control from the hijackers.
The attacks resulted in 2,977 fatalities, making it the single deadliest terrorist attack in human history. The event prompted major U.S. initiatives to combat terrorism, leading to the global "War on Terror," the invasion of Afghanistan, and the passage of new legislation like the USA PATRIOT Act. It also fundamentally altered national security protocols, particularly in air travel, and had a lasting impact on international relations and American foreign policy.
