noun
9 PM; 21:00
Used in 24-hour time notation; common in train schedules, business hours, and formal announcements.
最終バスは２１時です。
The last bus is at 9 PM.
会議は二十一時から始まります。
The meeting starts at 21:00.
午後９時 is the 12-hour equivalent, more common in everyday conversation.
Compound of 二十一 (にじゅういち, twenty-one) and 時 (じ, hour).