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.
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            Something deleterious and malignant as his touch.
                                                  --Hawthorne.
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   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:

Anthrax \An"thrax\ ([a^]n"thr[a^]ks), n. [L., fr. Gr. 'a`nqrax
   coal, carbuncle.]
   1. (Med.)
      (a) A carbuncle.
      (b) A malignant pustule.
          [1913 Webster]

   2. (Biol.) A microscopic, bacterial organism (Bacillus
      anthracis), resembling transparent rods. [See Illust.
      under Bacillus.]
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   3. An infectious disease of cattle and sheep. It is ascribed
      to the presence of a rod-shaped gram-positive bacterium
      (Bacillus anthracis), the spores of which constitute the
      contagious matter. It may be transmitted to man by
      inoculation. The spleen becomes greatly enlarged and
      filled with bacteria. Called also splenic fever.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Anthrax vaccine \An"thrax vac"cine\ (Veter.)
   A fluid vaccine obtained by growing a bacterium (Bacillus
   anthracis, formerly Bacterium anthracis) in beef broth. It
   is used to immunize animals, esp. cattle.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Bacillus anthracis
    n 1: a species of bacillus that causes anthrax in humans and in
         animals (cattle and swine and sheep and rabbits and mice
         and guinea pigs); can be used a bioweapon [syn: Bacillus
         anthracis, anthrax bacillus]