in or during the day regularly: They slept days rather than nights.
the interval of light between two successive nights; the time between sunrise and sunset: Since there was no artificial illumination, all activities had to be carried on during the day.
the light of day; daylight: The owl sleeps by day and feeds by night.
Astronomy. Also called mean solar day. a division of time equal to 24 hours and representing the average length of the period during which the earth makes one rotation on its axis. Also called solar day. a division of time equal to the time elapsed between two consecutive returns of the same terrestrial meridian to the sun. Also called civil day. a division of time equal to 24 hours but reckoned from one midnight to the next. Compare lunar day, sidereal day.
an analogous division of time for a planet other than the earth: the Martian day.