Source : Webster's Revised Unabridged Dictionary (1913)

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).

Caustic \Caus"tic\, Caustical \Caus"tic*al\, a. [L. caustucs,
   Ge. ?, fr. ? to burn. Cf. {Calm}, {Ink}.]
   1. Capable of destroying the texture of anything or eating
      away its substance by chemical action; burning; corrosive;
      searing.

   2. Severe; satirical; sharp; as, a caustic remark.

   {Caustic curve} (Optics), a curve to which the ray of light,
      reflected or refracted by another curve, are tangents, the
      reflecting or refracting curve and the luminous point
      being in one plane.

   {Caustic lime}. See under {Lime}.

   {Caustic potash}, {Caustic soda} (Chem.), the solid
      hydroxides potash, {KOH}, and soda, {NaOH}, or solutions
      of the same.

   {Caustic silver}, nitrate of silver, lunar caustic.

   {Caustic surface} (Optics), a surface to which rays reflected
      or refracted by another surface are tangents. Caustic
      curves and surfaces are called catacaustic when formed by
      reflection, and diacaustic when formed by refraction.

   Syn: Stinging; cutting; pungent; searching.