Definition of 9/11 in US English:

9/11

proper noun

  • An attack on September 11, 2001 in which two hijacked airliners were flown directly into the World Trade Center in New York City, causing the complete collapse of the twin towers, and a third airliner was flown into the Pentagon. A fourth aircraft was brought to the ground in Pennsylvania as a result of what is thought to be passenger intervention. The airliners were hijacked by Islamic fundamentalist terrorists believed to be involved with al-Qaeda.

    Also called September 11
    • ‘This guy thinks 9/11 was an inside job.’
    • ‘The biggest shift in the media landscape post-9/11 was the explosion of blogs and online citizen journalists. By Robert Andrews.’

Pronunciation

9/11

/ˌnaɪn ɪˈlɛvən/