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]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Severe; satirical; sharp; as, a caustic remark.
      [1913 Webster]

   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.
        [1913 Webster]