The Collaborative International Dictionary of English v.0.48:

malice \mal"ice\ (m[a^]l"[i^]s), n. [F. malice, fr. L. malitia,
   from malus bad, ill, evil, prob. orig., dirty, black; cf. Gr.
   me`las black, Skr. mala dirt. Cf. Mauger.]
   1. Enmity of heart; malevolence; ill will; a spirit
      delighting in harm or misfortune to another; a disposition
      to injure another; a malignant design of evil. "Nor set
      down aught in malice." --Shak.
      [1913 Webster]

            Envy, hatred, and malice are three distinct passions
            of the mind.                          --Ld. Holt.
      [1913 Webster]

   2. (Law) Any wicked or mischievous intention of the mind; a
      depraved inclination to mischief; an intention to vex,
      annoy, or injure another person, or to do a wrongful act
      without just cause or cause or excuse; a wanton disregard
      of the rights or safety of others; willfulness.
      [1913 Webster]

   Malice aforethought or Malice prepense, malice previously
      and deliberately entertained.
      [1913 Webster]

   Syn: Spite; ill will; malevolence; grudge; pique; bitterness;
        animosity; malignity; maliciousness; rancor; virulence.

   Usage: See Spite. -- Malevolence, Malignity,
          Malignancy. Malice is a stronger word than
          malevolence, which may imply only a desire that evil
          may befall another, while malice desires, and perhaps
          intends, to bring it about. Malignity is intense and
          deepseated malice. It implies a natural delight in
          hating and wronging others. One who is malignant must
          be both malevolent and malicious; but a man may be
          malicious without being malignant.
          [1913 Webster]

                Proud tyrants who maliciously destroy
                And ride o'er ruins with malignant joy.
                                                  --Somerville.
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                in some connections, malignity seems rather more
                pertinently applied to a radical depravity of
                nature, and malignancy to indications of this
                depravity, in temper and conduct in particular
                instances.                        --Cogan.
          [1913 Webster]

WordNet (r) 3.0 (2006):

malice aforethought
    n 1: (law) criminal intent; the thoughts and intentions behind a
         wrongful act (including knowledge that the act is illegal);
         often at issue in murder trials [syn: mens rea, malice
         aforethought]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MALICE AFORETHOUGHT, pleadings. In an indictment for murder, these words,
which have a technical force, must be used in charging the offence; for
without them, and the artificial phrase murder, the indictment will be taken
to charge manslaughter only. Fost. 424; Yelv. 205; 1 Chit. Cr. Law, *242,
and the authorities and cases there cited.
     2. Whenever malice aforethought is necessary to constitute the offence,
these words must be used in charging the crime in the indictment. 2 Chit.
Cr. Law, *787; 1 East, Pl. Or. 402. 2 Mason, R. 91.