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