Pronunciation: (in-fal'u-bul),[key] —adj. 1. absolutely trustworthy or sure: an infallible rule. 2. unfailing in effectiveness or operation; certain: an infallible remedy. 3. not fallible; exempt from liability to error, as persons, their judgment, or pronouncements: an infallible principle. 4. Rom. Cath. Ch.immune from fallacy or liability to error in expounding matters of faith or morals by virtue of the promise made by Christ to the Church.