资料来源 : 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.

资料来源 : WordNet®

lunar day
     n : the period of time taken for the moon to make one full
         rotation on its axis (about 27.3 sidereal days)