The Collaborative International Dictionary of English v.0.48:

Invasive \In*va"sive\, a. [LL. invasivus: cf. F. invasif. See
   Invade.]
   1. Tending to invade; characterized by invasion; aggressive.
      "Invasive war." --Hoole.
      [1913 Webster]

   2. (Med.) tending to spread, especially tending to intrude
      into healthy tissue; -- used mostly of tumors. [Narrower
      terms: malignant] PJC]

The Collaborative International Dictionary of English v.0.48:

malignant \ma*lig"nant\, a. [L. malignans, -antis, p. pr. of
   malignare, malignari, to do or make maliciously. See
   Malign, and cf. Benignant.]
   1. Disposed to do harm, inflict suffering, or cause distress;
      actuated by extreme malevolence or enmity; virulently
      inimical; bent on evil; malicious.
      [1913 Webster]

            A malignant and a turbaned Turk.      --Shak.
      [1913 Webster]

   2. Characterized or caused by evil intentions; pernicious.
      "Malignant care." --Macaulay.
      [1913 Webster]

            Some malignant power upon my life.    --Shak.
      [1913 Webster]

            Something deleterious and malignant as his touch.
                                                  --Hawthorne.
      [1913 Webster]

   3. (Med.) Tending to produce death; threatening a fatal
      issue; virulent; as, malignant diphtheria.
      [1913 Webster]

   Malignant pustule (Med.), a very contagious disease
      produced by infection of subcutaneous tissues with the
      bacterium Bacillus anthracis. It is transmitted to man
      from animals and is characterized by the formation, at the
      point of reception of the infection, of a vesicle or
      pustule which first enlarges and then breaks down into an
      unhealthy ulcer. It is marked by profound exhaustion and
      often fatal. The disease in animals is called charbon;
      in man it is called cutaneous anthrax, and formerly was
      sometimes called simply anthrax.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Malignant \Ma*lig"nant\, n.
   1. A man of extreme enmity or evil intentions. --Hooker.
      [1913 Webster]

   2. (Eng. Hist.) One of the adherents of Charles I. or Charles
      II.; -- so called by the opposite party.
      [1913 Webster]

WordNet (r) 3.0 (2006):

malignant
    adj 1: dangerous to health; characterized by progressive and
           uncontrolled growth (especially of a tumor) [ant:
           benign]

Moby Thesaurus II by Grady Ward, 1.0:

157 Moby Thesaurus words for "malignant":
   acrid, allergic, anemic, antagonistic, antipathetic, apoplectic,
   arthritic, atrocious, baleful, baneful, barbaric, barbarous,
   belligerent, bestial, bilious, bitchy, bitter, bloody, brutal,
   brutish, cancerous, catching, caustic, chlorotic, clashing,
   colicky, colliding, communicable, conflicting, consumptive,
   contagious, corroding, corrosive, corrupting, corruptive,
   counterproductive, cussed, damaging, deadly, death-bringing,
   deathful, deathly, deleterious, despiteful, destructive,
   detrimental, devilish, diabolical, disadvantageous, disserviceable,
   distressing, dropsical, dyspeptic, edematous, encephalitic,
   envenomed, epileptic, evil, fatal, feral, ferine, ferocious,
   fiendish, fierce, full of hate, harmful, hateful, hostile, hurtful,
   infectious, infective, inhuman, iniquitous, injurious, internecine,
   invidious, kill-crazy, killing, laryngitic, leprous, lethal,
   luetic, malarial, malefic, maleficent, malevolent, malicious,
   malign, mean, measly, mephitic, merciless, miasmal, miasmatic,
   miasmic, mischievous, mortal, murderous, nasty, nephritic,
   neuralgic, neuritic, noisome, noncivilized, noxious, ominous,
   ornery, palsied, paralytic, pernicious, pestiferous, pestilential,
   phthisic, pitiless, pleuritic, pneumonic, pocky, podagric,
   poisonous, prejudicial, quarrelsome, rachitic, rancorous,
   repugnant, rheumatic, rickety, ruthless, sanguinary, savage,
   scatheful, scorbutic, scrofulous, set against, sore, spiteful,
   tabetic, tabid, tameless, toxic, toxicant, toxiferous, tubercular,
   tuberculous, tumorigenic, tumorous, uncivilized, ungentle, untamed,
   venenate, veneniferous, venenous, venomous, vicious, virulent,
   vitriolic, wicked, wild