Also found in: Thesaurus.


1. Incapable of erring: an infallible guide; an infallible source of information.
2. Incapable of failing; certain: an infallible antidote; an infallible rule.
3. Roman Catholic Church Incapable of error in expounding doctrine on faith or morals.

[Middle English, from Medieval Latin īnfallibilis : Latin in-, not; see in-1 + Medieval Latin fallibilis, fallible; see fallible.]

in·fal′li·bil′i·ty, in·fal′li·ble·ness n.
in·fal′li·bly adv.