In·fal·li·bil·i·ty n.  The quality or state of being infallible, or exempt from error; inerrability.
    Infallibility is the highest perfection of the knowing faculty.   --Tillotson.
 Papal infallibility R. C. Ch., the dogma that the pope can not, when acting in his official character of supreme pontiff, err in defining a doctrine of Christian faith or rule of morals, to be held by the church. This was decreed by the Ecumenical Council at the Vatican, July 18, 1870.

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 In·fal·li·ble a.
 1. Not fallible; not capable of erring; entirely exempt from liability to mistake; unerring; inerrable.
 2. Not liable to fail, deceive, or disappoint; indubitable; sure; certain; as, infallible evidence; infallible success; an infallible remedy.
    To whom also he showed himself alive, after his passion, by many infallible proofs.   --Acts i. 3.
 3. R. C. Ch. Incapable of error in defining doctrines touching faith or morals. See Papal infallibility, under Infallibility.

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 Papal infallibility
      n : belief of the Roman Catholic Church that God protects the
          Pope from error when he speaks about faith or morality