The date of the attack on the Twin Towers and the Pentagon in the USA, September 11th, 2001. / (metonymically) The attack itself.

An event comparable to 9/11.

(CB slang) Location.

(CB slang) affirmative. / Message received, understood, acknowledged

Abbreviation of tenth.

(Britain) An examination, now mostly phased out, taken at age 11, to select the type of secondary education – either a grammar school or a secondary modern school.

(chiefly US, postpositive) Basic, beginner, starting from scratch.

A large number, particularly the size of an exhaustively long list of examples, uses, applications, etc.

Abbreviation of eleventh.

Alternative form of 10BASE5