The Collaborative International Dictionary of English v.0.48:

Incisive \In*ci"sive\, a. [Cf. F. incisif.]
   [1913 Webster]
   1. Having the quality of incising, cutting, or penetrating,
      as with a sharp instrument; cutting; hence, sharp; acute;
      sarcastic; biting; trenchant. "An incisive, high voice."
      --G. Eliot.
      [1913 Webster]

            And her incisive smile accrediting
            That treason of false witness in my blush. --Mrs.
                                                  Browning.
      [1913 Webster]

   2. (Anat.) Of or pertaining to the incisors; incisor; as, the
      incisive bones, the premaxillaries.
      [1913 Webster]

   3. Clearly and succinctly stated, with penetrating insight
      into the issue at hand; as, an incisive comment.
      [PJC]

WordNet (r) 3.0 (2006):

incisive
    adj 1: having or demonstrating ability to recognize or draw fine
           distinctions; "an acute observer of politics and
           politicians"; "incisive comments"; "icy knifelike
           reasoning"; "as sharp and incisive as the stroke of a
           fang"; "penetrating insight"; "frequent penetrative
           observations" [syn: acute, discriminating,
           incisive, keen, knifelike, penetrating,
           penetrative, piercing, sharp]
    2: suitable for cutting or piercing; "incisive teeth"

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "incisive":
   acerb, acerbate, acerbic, acid, acidic, acidulent, acidulous,
   acrid, acrimonious, active, acute, aggressive, animated,
   apperceptive, appercipient, argute, astringent, astute, biting,
   bitter, brisk, canny, caustic, clear-cut, cogent, corroding,
   corrosive, critical, cutting, cynical, discerning, double-edged,
   driving, dynamic, edged, effective, energetic, enterprising,
   enthusiastic, escharotic, farseeing, farsighted, fierce, forceful,
   forcible, foreseeing, foresighted, forethoughted, forethoughtful,
   full of pep, go-go, gutsy, harsh, hearty, imperative, impetuous,
   impressive, intense, ironical, keen, kinetic, lively, living,
   longheaded, longsighted, lusty, mettlesome, mordacious, mordant,
   nervous, penetrating, peppy, perceptive, percipient, perspicacious,
   perspicuous, piercing, poignant, powerful, provident, punchy,
   rigorous, robust, rough, sagacious, sarcastic, sardonic, scathing,
   scorching, sensational, severe, sharp, shrewd, sinewed, sinewy,
   slashing, smacking, snappy, spanking, spirited, stabbing, stinging,
   strenuous, strident, striking, stringent, strong, take-charge,
   take-over, tart, telling, terse, trenchant, understanding,
   vehement, vibrant, vigorous, violent, virulent, vital, vitriolic,
   vivacious, vivid, withering, zestful, zesty, zippy