The Collaborative International Dictionary of English v.0.48:

Precipitation \Pre*cip`i*ta"tion\, n. [L. praecipitatio: cf. F.
   pr['e]cipitation.]
   1. The act of precipitating, or the state of being
      precipitated, or thrown headlong.
      [1913 Webster]

            In peril of precipitation
            From off rock Tarpeian.               --Shak.
      [1913 Webster]

   2. A falling, flowing, or rushing downward with violence and
      rapidity.
      [1913 Webster]

            The hurry, precipitation, and rapid motion of the
            water, returning . . . towards the sea. --Woodward.
      [1913 Webster]

   3. Great hurry; rash, tumultuous haste; impetuosity. "The
      precipitation of inexperience." --Rambler.
      [1913 Webster]

   4. (Chem.) The act or process of precipitating from a
      solution.
      [1913 Webster]

   5. (Meteorology) A deposit on the earth of hail, mist, rain,
      sleet, or snow; also, the quantity of water deposited.

   Note: Deposits of dew, fog, and frost are not regarded by the
         United States Weather Bureau as precipitation. Sleet
         and snow are melted, and the record of precipitation
         shows the depth of the horizontal layers of water in
         hundredths of an inch or in millimeters.
         [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

precipitation
    n 1: the quantity of water falling to earth at a specific place
         within a specified period of time; "the storm brought
         several inches of precipitation"
    2: the process of forming a chemical precipitate
    3: the falling to earth of any form of water (rain or snow or
       hail or sleet or mist) [syn: precipitation, downfall]
    4: the act of casting down or falling headlong from a height
    5: an unexpected acceleration or hastening; "he is responsible
       for the precipitation of his own demise"
    6: overly eager speed (and possible carelessness); "he soon
       regretted his haste" [syn: haste, hastiness, hurry,
       hurriedness, precipitation]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "precipitation":
   Scotch mist, abruptness, air speed, alacrity, alluvion, alluvium,
   ash, blood rain, carelessness, celerity, cinder, clinker, deposit,
   deposition, deposits, desperateness, devil-may-careness, diluvium,
   dispatch, downcast, downpour, downthrow, draff, dregs, drizzle,
   dross, ember, evening mist, expedition, expeditiousness, fall,
   fastness, feces, feverishness, flight, flit, flurry, forwardness,
   froth, furiousness, gout of rain, ground speed, grounds, hail,
   haste, hastiness, heedlessness, hotheadedness, hurriedness, hurry,
   impatience, impetuosity, impetuousness, impulse, impulsiveness,
   instantaneousness, knots, lees, lightning speed, loess,
   miles per hour, mist, misty rain, mizzle, moisture, moraine,
   offscum, overeagerness, overenthusiasm, overhastiness, overthrow,
   overturn, overzealousness, patter, pitter-patter, precipitance,
   precipitancy, precipitate, precipitateness, precipitousness,
   precociousness, precocity, prematureness, prematurity, promptitude,
   promptness, quickness, rain, raindrop, rainfall, rainwater,
   rapidity, rashness, recklessness, round pace, rpm, rush, scoria,
   scum, sediment, sedimentation, settlings, sheet of rain, shower,
   showers, silt, sinter, slag, sleet, smut, snappiness, snow,
   snowfall, soot, speed, speediness, splatter, sprinkle, sublimate,
   suddenness, swift rate, swiftness, unfrozen hydrometeor,
   untimeliness, velocity, wantonness, wet, wildness