The Collaborative International Dictionary of English v.0.48:

Inevitability \In*ev`i*ta*bil"i*ty\, n. [Cf. F.
   in['e]vitabilit['e].]
   Impossibility to be avoided or shunned; inevitableness.
   --Shelford.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inevitability
    n 1: the quality of being unavoidable [syn: inevitability,
         inevitableness]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "inevitability":
   Friday, Friday the thirteenth, absolute certainty, absoluteness,
   act of God, appointed lot, assurance, assuredness,
   astral influences, astrology, book of fate, certain knowledge,
   certainness, certainty, certitude, coaction, compulsion,
   compulsiveness, constellation, constraint, cup, dead certainty,
   definiteness, destination, destiny, determinacy, determinateness,
   dies funestis, doom, end, enforcement, fatality, fate, fatefulness,
   force majeure, forcing, foredoom, fortune, future, ides of March,
   indefeasibility, ineluctability, inerrability, inerrancy,
   inescapableness, inevasibleness, inevitable accident,
   inevitableness, inexorability, infallibilism, infallibility,
   inflexibility, irresistibility, irrevocability, kismet, lot, moira,
   necessity, nonambiguity, noncontingency, obligation, obligement,
   planets, portion, positiveness, predestination, predetermination,
   probatum, proved fact, relentlessness, restraint, stars, sureness,
   surety, truth, unambiguity, unavoidable casualty, unavoidableness,
   uncontrollability, undeflectability, unequivocalness, univocity,
   unlucky day, unmistakableness, unpreventability, unyieldingness,
   vis major, weird, wheel of fortune, will of Heaven