The eleventh item in a series is the one that you count as number eleven.
例句
We were working on the eleventh floor.我們在11層辦公。
英漢詞典釋義
num.
第十一
an eleventh-hour decision最后一刻作的決定He arrived there at the eleventh hour.他剛好到達那里。The antique is assigned to the eleventh Century.這種古玩被認為是11世紀的遺物。
最后關頭；最后一刻
Our pianist had fallen ill, and then, at the eleventh hour, when we thought we’d have to cancel the performance, Jill offered to replace him.我們的鋼琴演奏者病倒了，在最后關頭，當我們以為不得不取消表演時， 吉爾表示愿意代替他演出。
英英詞典釋義
Noun
1. position 11 in a countable series of things
Adjective
1. coming next after the tenth and just before the twelfth in position