Akademik

the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs

(also September 11th) n. September 11, 2001. On this date, two hijacked commercial airliners were flown into the World Trade Center in lower Manhattan. A third hijacked airliner was crashed into the Pentagon, and a fourth went down in a field in Pennsylvania

9/11 8 [9/11] [ˌnaɪn ɪˈlevn] [ˌnaɪn ɪˈlevn] noun

the abbreviation for the date September 11, 2001, when ↑terrorists flew planes into the World Trade Center in New York, the Pentagon in Washington, D.C., and a field in Pennsylvania, killing thousands of people

People in the US write dates in the form of the month followed by the day, whereas in Britain the month follows the day. However, the US form is used in both countries to refer to the day of these attacks. The abbreviation often appears in phrases describing people or things connected with them, for example 9/11 families (= families affected by the attacks), the 9/11 Commission, the post-9/11 world.