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]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Optics) A caustic curve or caustic surface.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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"; "a vitriolic
           critique" [syn: acerb, acerbic, acid, acrid,
           bitter, blistering, caustic, sulfurous,
           sulphurous, virulent, vitriolic]
    2: of a substance, especially a strong acid; capable of
       destroying or eating away by chemical action [syn: caustic,
       corrosive, erosive, vitriolic, mordant]
    n 1: any chemical substance that burns or destroys living tissue

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "caustic":
   Rabelaisian, acerb, acerbate, acerbic, acid, acidic, acidulent,
   acidulous, acrid, acrimonious, actual cautery, acute, amaroidal,
   angry, antagonistic, antipathetic, arc, asperous, astringent,
   belligerent, biting, bitter, bitter as gall, bow, brand,
   brand iron, branding iron, brisk, burning, catacaustic, catenary,
   cauter, cauterant, cauterizer, cautery, choleric, circle, clashing,
   coarse, colliding, conchoid, conflicting, corroding, corrosive,
   crisp, critical, crook, curl, curve, cutting, cynical, despiteful,
   destructive, diacaustic, discontented, double-edged, dry, edged,
   electrocautery, ellipse, embittered, escharotic, feeling evil,
   festoon, fierce, full of hate, hard, harsh, hateful, hook, hostile,
   hot iron, hyperbola, incisive, ironic, irritating, keen, lituus,
   lunar caustic, malevolent, malicious, malignant, mordacious,
   mordant, moxa, nose-tickling, out of humor, out of sorts,
   out of temper, parabola, penetrating, piercing, piquant, pithy,
   poignant, potential cautery, pungent, quarrelsome, radium,
   rancorous, rankled, repugnant, resentful, resenting, rigorous,
   rough, salty, sarcastic, sardonic, satiric, scathing, scorching,
   set against, severe, sharp, sinus, sore, sour, spiteful, splenetic,
   stabbing, stewing, stinging, strident, stringent, succinct, tart,
   terse, tracery, trenchant, vehement, venomous, violent, virulent,
   vitriolic, withering