SEPT. 11

 Dictionary entry overview: What does Sept. 11 mean? 

• SEPT. 11 (noun)
  The noun SEPT. 11 has 1 sense:

1. the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs

  Familiarity information: SEPT. 11 used as a noun is very rare.

 Dictionary entry details 

• SEPT. 11 (noun)

Sense 1

Meaning:

The day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs

Classified under:

Nouns denoting time and temporal relations

Synonyms:

9-11; 9/11; Sep 11; Sept. 11; September 11

Instance hypernyms:

terrorist attack (a surprise attack involving the deliberate use of violence against civilians in the hope of attaining political or religious aims)

Domain category:

act of terrorism; terrorism; terrorist act (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)

Holonyms ("Sept. 11" is a part of...):

Sep; Sept; September (the month following August and preceding October)

 Context examples 

The ozone hole reached its peak extent on Sept. 11, covering an area about two and a half times the size of the United States, and then declined through the remainder of September and into October.

(Warmth in the Antarctic stratosphere helped limit the size of the ozone hole in 2017 to the smallest observed since 1988, NOAA)