adj.1 incapable of error.
2 (of a method, test, proof, etc.) unfailing; sure to succeed.
3 RC Ch. (of the Pope) unable to err in pronouncing dogma as doctrinally defined.
Derivatives:
infallibility n. infallibly adv.
Etymology: ME f. F infaillible or LL infallibilis (as IN-(1), FALLIBLE)