Definition of Papal Infallibility
From the Vatican Council : 1869 - 1870
Session IV (July 18, 1870)
Dogmatic Constitution I on the Church of Christ
But since in this very age, in which the salutary efficacy of the apostolic duty is especially required, not a few are found who disparage its authority, We deem it most necessary to assert solemnly the prerogative which the Only-begotten Son of God deigned to enjoin with the highest pastoral office.
And so We, adhering faithfully to the tradition received from the beginning of the Christian faith, to the glory of God, our Saviour, the elevation of the Catholic religion and the salvation of Christian peoples, with the approbation of the sacred Council, teach and explain that the dogma has been divinely revealed, that the Roman Pontiff, when he speaks ex cathedra, that is, when carrying out the duty of the pastor and teacher of all Christians in accord with his supreme apostolic authority he explains a doctrine of faith or morals to be held by the universal Church, through the divine assistance promised him in blessed Peter, operates with that infallibility with which the divine Redeemer wished that His church be instructed in defining doctrine on faith and morals; and so such definitions of the Roman Pontiff himself , but not from the consensus of the Church, ARE UNALTERABLE.
[Canon]. But if anyone presumes to contradict this definition of Ours, which may God forbid: let him be anathema.