The Collaborative International Dictionary of English v.0.48:

Mean \Mean\, a. [OE. mene, OF. meiien, F. moyen, fr. L. medianus
   that is in the middle, fr. medius; akin to E. mid. See
   Mid.]
   1. Occupying a middle position; middle; being about midway
      between extremes.
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            Being of middle age and a mean stature. --Sir. P.
                                                  Sidney.
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   2. Intermediate in excellence of any kind.
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            According to the fittest style of lofty, mean, or
            lowly.                                --Milton.
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   3. (Math.) Average; having an intermediate value between two
      extremes, or between the several successive values of a
      variable quantity during one cycle of variation; as, mean
      distance; mean motion; mean solar day.
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   Mean distance (of a planet from the sun) (Astron.), the
      average of the distances throughout one revolution of the
      planet, equivalent to the semi-major axis of the orbit.

   Mean error (Math. Phys.), the average error of a number of
      observations found by taking the mean value of the
      positive and negative errors without regard to sign.

   Mean-square error, or Error of the mean square (Math.
      Phys.), the error the square of which is the mean of the
      squares of all the errors; -- called also, mean square
      deviation, mean error.

   Mean line. (Crystallog.) Same as Bisectrix.

   Mean noon, noon as determined by mean time.

   Mean proportional (between two numbers) (Math.), the square
      root of their product.

   Mean sun, a fictitious sun supposed to move uniformly in
      the equator so as to be on the meridian each day at mean
      noon.

   Mean time, time as measured by an equable motion, as of a
      perfect clock, or as reckoned on the supposition that all
      the days of the year are of a mean or uniform length, in
      contradistinction from apparent time, or that actually
      indicated by the sun, and from sidereal time, or that
      measured by the stars.
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WordNet (r) 3.0 (2006):

mean sun
    n 1: a theoretical sun that moves along the celestial equator at
         a constant speed and completes its annual course in the
         same amount of time the real sun takes at variable speeds