9-11 is a noun meaning "the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs". It carries one recorded sense.
the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs
terrorism
Definitions, synonyms and semantic relations for 9-11 come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.