Source : Webster's Revised Unabridged Dictionary (1913)

Day \Day\, n. [OE. day, dai,, dei, AS. d[ae]g; akin to OS., D.,
   Dan., & Sw. dag, G, tag, Icel. dagr, Goth. dags; cf. Skr. dah
   (for dhagh ?) to burn. [root]69. Cf. {Dawn}.]
   1. The time of light, or interval between one night and the
      next; the time between sunrise and sunset, or from dawn to
      darkness; hence, the light; sunshine.

   2. The period of the earth's revolution on its axis. --
      ordinarily divided into twenty-four hours. It is measured
      by the interval between two successive transits of a
      celestial body over the same meridian, and takes a
      specific name from that of the body. Thus, if this is the
      sun, the day (the interval between two successive transits
      of the sun's center over the same meridian) is called a
      {solar day}; if it is a star, a {sidereal day}; if it is
      the moon, a {lunar day}. See {Civil day}, {Sidereal day},
      below.

   3. Those hours, or the daily recurring period, allotted by
      usage or law for work.

   4. A specified time or period; time, considered with
      reference to the existence or prominence of a person or
      thing; age; time.

            A man who was great among the Hellenes of his day.
                                                  --Jowett
                                                  (Thucyd. )

            If my debtors do not keep their day, . . . I must
            with patience all the terms attend.   --Dryden.

   5. (Preceded by the) Some day in particular, as some day of
      contest, some anniversary, etc.

            The field of Agincourt, Fought on the day of Crispin
            Crispianus.                           --Shak.

            His name struck fear, his conduct won the day.
                                                  --Roscommon.

   Note: Day is much used in self-explaining compounds; as,
         daybreak, daylight, workday, etc.

Source : WordNet®

day
     n 1: time for Earth to make a complete rotation on its axis; "two
          days later they left"; "they put on two performances
          every day"; "there are 30,000 passengers per day" [syn:
          {twenty-four hours}, {solar day}, {mean solar day}]
     2: some point or period in time; "it should arrive any day
        now"; "after that day she never trusted him again"; "those
        were the days"; "these days it is not unusual"
     3: the time after sunrise and before sunset while it is light
        outside; "the dawn turned night into day"; "it is easier
        to make the repairs in the daytime" [syn: {daytime}, {daylight}]
        [ant: {night}]
     4: a day assigned to a particular purpose or observance;
        "Mother's Day"
     5: the recurring hours when you are not sleeping (especially
        those when you are working); "my day began early this
        morning"; "it was a busy day on the stock exchange"; "she
        called it a day and went to bed"
     6: an era of existence or influence; "in the day of the
        dinosaurs"; "in the days of the Roman Empire"; "in the
        days of sailing ships"; "he was a successful pianist in
        his day"
     7: a period of opportunity; "he deserves his day in court";
        "every dog has his day"
     8: the period of time taken by a particular planet (e.g. Mars)
        to make a complete rotation on its axis; "how long is a
        day on Jupiter?"
     9: the time for one complete rotation of the earth relative to
        a particular star, about 4 minutes shorter than a mean
        solar day [syn: {sidereal day}]
     10: United States writer best known for his autobiographical
         works (1874-1935) [syn: {Clarence Day}, {Clarence Shepard
         Day Jr.}]