The Collaborative International Dictionary of English v.0.48:

antagonism \an*tag"o*nism\ ([a^]n*t[a^]g"[-o]*n[i^]z'm), n. [Gr.
   'antagw`nisma, fr. 'antagwni`zesqai to struggle against;
   'anti` against + 'agwni`zesqai to contend or struggle, 'agw`n
   contest: cf. F. antagonisme. See Agony.]
   Opposition of action; counteraction or contrariety of things
   or principles.
   [1913 Webster]

   Note: We speak of antagonism between two things, to or
         against a thing, and sometimes with a thing.
         [1913 Webster]

WordNet (r) 3.0 (2006):

antagonism
    n 1: a state of deep-seated ill-will [syn: hostility,
         enmity, antagonism]
    2: the relation between opposing principles or forces or
       factors; "the inherent antagonism of capitalism and
       socialism"
    3: an actively expressed feeling of dislike and hostility
    4: (biochemistry) interference in or inhibition of the
       physiological action of a chemical substance by another
       having a similar structure