in·fal·li·ble 
(ĭn-fălə-bəl)
(ĭn-fălə-bəl)adj.
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·bili·ty, in·falli·ble·ness n.
in·falli·bly adv.