Source : Webster's Revised Unabridged Dictionary (1913)

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.

Caustic \Cau"stic\, n. [L. causticum (sc. medicamentum). See
   {Caustic}, a.]
   1. Any substance or means which, applied to animal or other
      organic tissue, burns, corrodes, or destroys it by
      chemical action; an escharotic.

   2. (Optics) A caustic curve or caustic surface.

Source : WordNet®

caustic
     adj 1: harsh or corrosive in tone; "an acerbic tone piercing
            otherwise flowery prose"; "a barrage of acid
            comments"; "her acrid remarks make her many enemies";
            "bitter words"; "blistering criticism"; "caustic jokes
            about political assassination, talk-show hosts and
            medical ethics"; "a sulfurous denunciation" [syn: {acerb},
             {acerbic}, {acid}, {acrid}, {bitter}, {blistering}, {sulfurous},
             {sulphurous}, {venomous}, {virulent}, {vitriolic}]
     2: of a substance, especially a strong acid; capable of
        destroying or eating away by chemical action [syn: {corrosive},
         {erosive}, {vitriolic}]

caustic
     n : any chemical substance that burns or destroys living tissue