› a period of 24 hours, esp. from 12 o’clock one night to 12 o’clock the next night, or the part of this period after the sun rises but before it goes down, when there is light: My husband picks up our son every day after school. In summer the days are longer and we have cookouts in the backyard. We leave on vacation the day after tomorrow. › A day is also the part of a period of 24 hours that you spend at work: He’s been working 12-hour days this week. › Days can mean a long period of time: In those days (= that period in history), people had large families.