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.
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            The crescent moon, the diadem of night. --Cowper.
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   2. A secondary planet, or satellite, revolving about any
      member of the solar system; as, the moons of Jupiter or
      Saturn.
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   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.
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   4. (Fort.) A crescentlike outwork. See Half-moon.
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   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.
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WordNet (r) 3.0 (2006):

moon trefoil
    n 1: evergreen shrub of southern European highlands having downy
         foliage and a succession of yellow flowers throughout the
         summer followed by curious snail-shaped pods [syn: moon
         trefoil, Medicago arborea]