http://dictionary.reference.com/browse/day
1. 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.
2. the light of day; daylight: The owl sleeps by day and feeds by night.
3. Astronomy. a. 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.
b. 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.
c. 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. 4. an analogous division of time for a planet other than the earth: the Martian day.
5. the portion of a day allotted to work: an eight-hour day.
6. a day on which something occurs: the day we met.
7. (often initial capital letter) a day assigned to a particular purpose or observance: New Year's Day.
8. a time considered as propitious or opportune: His day will come.
9. a day of contest or the contest itself: to win the day.
10. Often, days. a particular time or period: the present day; in days of old.
11. Usually, days. period of life or activity: His days are numbered.
12. period of existence, power, or influence: in the day of the dinosaurs.
13. light 1 (def. 19a).
—Idioms
14. call it a day, to stop one's activity for the day or for the present; quit temporarily: After rewriting the paper, she decided to call it a day.
15. 'day in', 'day out', 'every day without fail'; regularly: They endured the noise and dirt of the city day in, day out. Also, 'day in and day out.'
Origin: bef. 950; ME; OE dæg; c. G Tag
Also, I also like to talk about the one the main dictionaries seem to like to omit:
16. The 48-hour time period in which a particular named 'day' exists on the World somewhere. Example: Since the time the sun rose West of the International Date Line On 27 Mar 2008 until this is posted on BB, 42 hours have passed. Even so there is left 6 hours of the day: Friday 27 Mar 2008.