The Collaborative International Dictionary of English v.0.48:

hostilities \hostilities\ n. pl.
   1. a legal state created by a declaration of war and ended by
      official declaration, during which the international rules
      of war apply.

   Syn: war, state of war, hot war.
        [WordNet 1.5]

              We have showed ourselves generous adversaries . .
              . and have carried on even our hostilities with
              humanity.                           --Atterbury.
        [1913 Webster]

   2. acts of overt warfare.

   Syn: belligerency.
        [WordNet 1.5]

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):

hostilities
    n 1: fighting; acts of overt warfare; "the outbreak of
         hostilities" [syn: hostilities, belligerency]