The Collaborative International Dictionary of English v.0.48:

Hostility \Hos*til"i*ty\, n.; pl. Hostilities. [L. hostilitas:
   cf. F. hostilit['e].]
   1. State of being hostile; public or private enemy;
      unfriendliness; animosity.
      [1913 Webster]

            Hostility being thus suspended with France.
                                                  --Hayward.
      [1913 Webster]

   2. An act of an open enemy; a hostile deed; especially in the
      plural, acts of warfare; attacks of an enemy. See
      hostilities
      [1913 Webster]

            He who proceeds to wanton hostility, often provokes
            an enemy where he might have a friend. --Crabb.

   Syn: Animosity; enmity; opposition; violence; aggression;
        contention; warfare.
        [1913 Webster]

WordNet (r) 3.0 (2006):

hostility
    n 1: a hostile (very unfriendly) disposition; "he could not
         conceal his hostility" [syn: hostility, ill will]
    2: a state of deep-seated ill-will [syn: hostility, enmity,
       antagonism]
    3: the feeling of a hostile person; "he could no longer contain
       his hostility" [syn: hostility, enmity, ill will]
    4: violent action that is hostile and usually unprovoked [syn:
       aggression, hostility]

The Devil's Dictionary (1881-1906):

HOSTILITY, n.  A peculiarly sharp and specially applied sense of the
earth's overpopulation.  Hostility is classified as active and
passive; as (respectively) the feeling of a woman for her female
friends, and that which she entertains for all the rest of her sex.