The Collaborative International Dictionary of English v.0.48:

Potash \Pot"ash`\, n. [Pot + ash.] (Chem.)
   (a) The hydroxide of potassium hydrate, a hard white brittle
       substance, KOH, having strong caustic and alkaline
       properties; -- hence called also caustic potash.
   (b) The impure potassium carbonate obtained by leaching wood
       ashes, either as a strong solution (lye), or as a white
       crystalline (pearlash).
       [1913 Webster]

WordNet (r) 3.0 (2006):

potash
    n 1: a potassium compound often used in agriculture and industry
         [syn: potash, caustic potash, potassium hydroxide]