Noon is twelve o'clock in the middle of the day. Many people sit down to eat lunch right at noon.
You can also call noon midday, and its opposite is midnight — also twelve o'clock, but falling in the middle of the night. The word noon comes from a Latin root, nona hora, or "ninth hour." In medieval times, noon fell at three PM, nine hours after a monk's traditional rising hour of six o'clock in the morning. Over time, as noon came to be synonymous in English with midday, its timing changed to twelve PM.