Twelve o'clock. Depending on context it can mean noon (12:00 PM) or midnight (12:00 AM). Pronounced 'jūniji' in speech.
昼休みは１２時から始まる。
Lunch break starts at twelve o'clock.
映画は１２時に終わる予定です。
The movie is scheduled to end at twelve o'clock.
新年は１２時に花火が上がった。
Fireworks went up at twelve o'clock to mark the New Year.
電車は深夜の１２時にはもう本数が少ない。
There are already fewer trains by midnight at twelve o'clock.