noun
twelve o'clock; midnight; noon
Means 0:00 or twelve o'clock in time expressions. In modern 24-hour usage it most often means midnight; 午前零時 is midnight and 午後零時 can mean noon.
午前零時に日付が変わる。
The date changes at midnight.
受付は午後零時から始まります。
Registration starts at noon.
Compound of 零, conventionally used here for zero, and 時, hour or time; the spelling directly reflects the idea of 'zero o'clock.'