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).

Source: Webster's Revised Unabridged Dictionary (1913)