The Collaborative International Dictionary of English v.0.48:

Invincibility \In*vin`ci*bil"i*ty\, n. [Cf. F.
   invincibilit['e].]
   The quality or state of being invincible; invincibleness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

invincibility
    n 1: the property being difficult or impossible to defeat [syn:
         indomitability, invincibility]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "invincibility":
   authenticity, authoritativeness, calculability, deathlessness,
   dependability, faithworthiness, firmness, immortality,
   impenetrability, imperishability, impregnability, incorruptibility,
   indelibility, indestructibility, indomitability, ineffaceability,
   ineradicability, inerasableness, inexpugnability, insuperability,
   inviolability, invulnerability, irresistibility, predictability,
   reliability, secureness, security, solidity, soundness, stability,
   staunchness, steadfastness, steadiness, substantiality,
   trustworthiness, unassailability, unattackableness, unbeatableness,
   unconquerableness, unerringness, validity