The Collaborative International Dictionary of English v.0.48:

Moon \Moon\ (m[=oo]n), n. [OE. mone, AS. m[=o]na; akin to D.
   maan, OS. & OHG. m[=a]no, G. mond, Icel. m[=a]ni, Dan. maane,
   Sw. m[*a]ne, Goth. m[=e]na, Lith. men[*u], L. mensis month,
   Gr. mh`nh moon, mh`n month, Skr. m[=a]s moon, month; prob.
   from a root meaning to measure (cf. Skr. m[=a] to measure),
   from its serving to measure the time. [root]271. Cf. Mete
   to measure, Menses, Monday, Month.]
   1. The celestial orb which revolves round the earth; the
      satellite of the earth; a secondary planet, whose light,
      borrowed from the sun, is reflected to the earth, and
      serves to dispel the darkness of night. The diameter of
      the moon is 2,160 miles, its mean distance from the earth
      is 240,000 miles, and its mass is one eightieth that of
      the earth. See Lunar month, under Month.
      [1913 Webster]

            The crescent moon, the diadem of night. --Cowper.
      [1913 Webster]

   2. A secondary planet, or satellite, revolving about any
      member of the solar system; as, the moons of Jupiter or
      Saturn.
      [1913 Webster]

   3. The time occupied by the moon in making one revolution in
      her orbit; a month; as, it's been many moons since I last
      visited Washington. --Shak.
      [1913 Webster]

   4. (Fort.) A crescentlike outwork. See Half-moon.
      [1913 Webster]

   5. The deliberately exposed naked buttocks. [slang]
      [PJC]

   Moon blindness.
      (a) (Far.) A kind of ophthalmia liable to recur at
          intervals of three or four weeks.
      (b) (Med.) Hemeralopia.

   Moon dial, a dial used to indicate time by moonlight.

   Moon face, a round face like a full moon.

   Moon madness, lunacy. [Poetic]

   Moon month, a lunar month.

   Moon trefoil (Bot.), a shrubby species of medic (Medicago
      arborea). See Medic.

   Moon year, a lunar year, consisting of lunar months, being
      sometimes twelve and sometimes thirteen.

   blue moon, see blue moon in the vocabulary.

   many moons, a long time.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

blue moon \blue moon"\
   The second full moon occurring in the same month; -- derived
   from the expression once in a blue moon; as, we had a blue
   moon on January 31, 1999, and another in March.
   [PJC]

   once in a blue moon very rarely; -- from the observation
      that the moon rarely has a bluish tint.

   Note: In 1980, the phrase blue moon was applied to the
         second full moon of any month, because this phenomenon
         is unusual. This new meaning for blue moon, a long
         time, used primarily in the phrase once in a blue moon,
         has become widely accepted.
         [PJC]

WordNet (r) 3.0 (2006):

blue moon
    n 1: a long time; "something that happens once in blue moon
         almost never happens"