What is Day?

Day definition and meaning on Dictionary terms:
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.See also lunar day, sidereal day.

an analogous division of time for a planet other than the earth: the Martian day.
the portion of a day allotted to work: an eight-hour day.
a day on which something occurs: the day we met.

(often initial capital letter) a day assigned to a particular purpose or observance: New Year’s Day.
a time considered as propitious or opportune: His day will come.
a day of contest or the contest itself: to win the day.
Often days. a particular time or period: the present day; in days of old.
Usually days. period of life or activity: His days are numbered.
period of existence, power, or influence: in the day of the dinosaurs.
Architecture. light1(def 19a).