the period of time, the calendar day, of 24 hours' duration reckoned from one midnight to the next
2.
a.
the period of light between sunrise and sunset, as distinguished from the night
b.
(as modifier)
the day shift
3.
the part of a day occupied with regular activity, esp work
he took a day off
4. (sometimes plural)
a period or point in time
he was a good singer in his day
in days gone by
any day now
5.
the period of time, the sidereal day, during which the earth makes one complete revolution on its axis relative to a particular star. The mean sidereal day lasts 23 hours 56 minutes 4.1 seconds of the mean solar day
6.
the period of time, the solar day, during which the earth makes one complete revolutionon its axis relative to the sun. The mean solar day is the average length of the apparent solar day and is some four minutes (3 minutes 56.5 seconds of sidereal time) longer than the sidereal day
7.
the period of time taken by a specified planet to make one complete rotation on its axis
the Martian day
8. (often capital)
a day designated for a special observance, esp a holiday
Christmas Day
9. all in a day's work
10. at the end of the day
11. back in the day
12. day of rest
13. end one's days
14. every dog has its day
15. in this day and age
16. it's early days
17. late in the day
18. that will be the day
19.
a time of success, recognition, power, etc
his day will soon come
20.
a struggle or issue at hand
the day is lost
21.
a.
the ground surface over a mine
b.
(as modifier)
the day level
22. from day to day
23. call it a day
24. day after day
25. day by day
26. day in, day out
27. from Day 1
28. one of these days
29. (modifier)
of, relating to, or occurring in the day
the day shift
▶ Related adjective: diurnal
Word origin
Old English dæg; related to Old High German tag, Old Norse dagr