The Collaborative International Dictionary of English v.0.48:

Mala \Ma"la\, n. pl.; pl. of Malum. [L.]
   Evils; wrongs; offenses against right and law.
   [1913 Webster]

   Mala in se [L.] (Law), offenses which are such from their
      own nature, at common law, irrespective of statute.

   Mala prohibita [L.] (Law), offenses prohibited by statute,
      as distinguished from mala in se, which are offenses at
      common law.
      [1913 Webster]

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

MALA PROHIBITA. Those things which are prohibited by law, and therefore
unlawful.
     2. A distinction was formerly made in respect of contracts, between
mala prohibita and mala in se; but that distinction has been exploded, and,
it is now established that when the provisions of an act of the legislature
have for their object the protection of the public, it makes no difference
with respect to contracts, whether the thing be prohibited absolutely or
under a penalty. 5 B. & A 5, 340; 10 B. & C. 98; 3 Stark. 61; 13 Pick. 518;
2 Bing. N. C. 636, 646.