The Collaborative International Dictionary of English v.0.48:

911 \9/11\ (n[imac]n" [-e]*l[e^]v"[e^]n) n.
   September 11, 2001 -- the day when two airplanes hijacked by
   Islamic terrorists were deliberately crashed into the "twin
   towers" (the two 110-storey skyscrapers of the World Trade
   Center) in New York City, causing those buildings to collapse
   and much other damage in the nearby area. Another plane was
   crashed into the Pentagon near Washington. In total, the
   attacks caused nearly 3,000 deaths. This was the worst
   incident of terrorism inside the United States to date. The
   abbreviated date 9/11 is now taken to refer to that specific
   day and the terrorist incidents that occured that day. It is
   used especially in the phrase "the events of 9/11".
   [PJC]

WordNet (r) 3.0 (2006):

9-11
    n 1: the day in 2001 when Arab suicide bombers hijacked United
         States airliners and used them as bombs [syn: 9/11,
         9-11, September 11, Sept. 11, Sep 11]