The Collaborative International Dictionary of English v.0.48:

Effloresce \Ef`flo*resce"\, v. i. [imp. & p. p. Effloresced;
   p. pr. & vb. n. Efflorescing.] [L. efflorescere to bloom,
   blossom; ex + florescere to begin to blossom, incho., fr.
   florere to blossom, fr. flos a flower. See Flower.]
   1. To blossom forth. --Carlyle.
      [1913 Webster]

   2. (Chem.) To change on the surface, or throughout, to a
      whitish, mealy, or crystalline powder, from a gradual
      decomposition, esp. from the loss of water, on simple
      exposure to the air; as, Glauber's salts, and many others,
      effloresce.
      [1913 Webster]

   3. To become covered with a whitish crust or light
      crystallization, from a slow chemical change between some
      of the ingredients of the matter covered and an acid
      proceeding commonly from an external source; as, the walls
      of limestone caverns sometimes effloresce with nitrate of
      calcium in consequence of the action of nitric acid formed
      in the atmosphere.
      [1913 Webster]

WordNet (r) 3.0 (2006):

effloresce
    v 1: come into or as if into flower; "These manifestations
         effloresced in the past" [syn: effloresce, burst forth]
    2: assume crystalline form; become crystallized [syn:
       crystallize, crystalize, crystalise, effloresce]
    3: become encrusted with crystals due to evaporation