Definition of September 11 in English:

September 11

(also 9/11)

proper noun

  • September 11, 2001. On this date, two hijacked commercial airliners were flown into the World Trade Center in New York City. A third hijacked airliner was crashed into the Pentagon, and a fourth went down in a field in Pennsylvania. The airliners were hijacked by Islamic fundamentalist terrorists believed to be involved with al-Qaeda.