The Collaborative International Dictionary of English v.0.48:

Unavoidable \Un`a*void"a*ble\, a.
   1. Not avoidable; incapable of being shunned or prevented;
      inevitable; necessary; as, unavoidable troubles.
      [1913 Webster]

   2. (Law) Not voidable; incapable of being made null or void.
      --Blackstone.
      [1913 Webster]

   Unavoidable hemorrhage (Med.), hemorrhage produced by the
      afterbirth, or placenta, being situated over the mouth of
      the womb so as to require detachment before the child can
      be born.
      [1913 Webster] -- Un`a*void"a*ble*ness, n. --
      Un`a*void"a*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unavoidable
    adj 1: impossible to avoid or evade:"inescapable conclusion";
           "an ineluctable destiny"; "an unavoidable accident" [syn:
           ineluctable, inescapable, unavoidable]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "unavoidable":
   certain, definite, destined, determined, fated, fateful, fixed,
   indefeasible, ineluctable, ineludible, inescapable, inevasible,
   inevitable, inexorable, inflexible, irresistible, irrevocable,
   necessary, predestined, predetermined, relentless, resistless,
   settled, sure, sure as death, sure as fate, unalterable,
   unchangeable, uncontrollable, undeflectable, unevadable,
   unpreventable, unstoppable, unyielding