One of the times on the clock: eleven o'clock (juuichi-ji). Context indicates whether it is a.m. or p.m.
飛行機は１１時に出発する予定だ。
The plane is scheduled to depart at eleven o'clock.
昼の１１時はレストランが混み始める時間だ。
11 a.m. is when restaurants start getting busy.
友達と１１時に駅で待ち合わせた。
I arranged to meet my friend at the station at eleven.