Sep 11 vs act of terrorism

Sep 11 and act of terrorism both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sep 11	Yes	No	No	No
act of terrorism	Yes	No	No	No

Sep 11 (noun)	act of terrorism (noun)
the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs	the calculated use of violence (or the threat of violence) against civilians in order to attain goals that are political or religious or ideological in nature; this is done through intimidation or coercion or instilling fear