9-11 9-11 /naɪn ɪlɛvən/

noun

1. The day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs  ( 9-11 , 9/11 , Sep 11 , Sept. 11 , September 11 , September 11 attacks )
   the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs