Ex Cathedra Literally "from the chair", a theological term which signifies authoritative teaching and is more particularly applied to the definitions given by the Roman pontiff. Originally the name of the seat occupied by a professor or a bishop, cathedra was used later on to denote the magisterium, or teaching authority. The phrase ex cathedra occurs in the writings of the medieval theologians, and more frequently in the discussions which arose after the Reformation in regard to the papal prerogatives. But its present meaning was formally determined by the Vatican Council, Sess. IV, Const. de Ecclesiâ Christi, c. iv: "We teach and define that it is a dogma Divinely revealed that the Roman pontiff when he speaks ex cathedra, that is when in discharge of the office of pastor and doctor of all Christians, by virtue of his supreme Apostolic authority, he defines a doctrine regarding faith or morals to be held by the universal Church, by the Divine assistance promised to him in Blessed Peter, is possessed of that infallibility with which the Divine Redeemer willed that his Church should be endowed in defining doctrine regarding faith or morals, and that therefore such definitions of the Roman pontiff are of themselves and not from the consent of the Church irreformable." (See INFALLIBILITY; POPE.) Meaning of Infallibility ect,read thie link-> http://www.newadvent.org/cathen/07790a.htm As you say madd as far as I know the popes have only done it a couple times in history. doctrine on papal infallibility as defined by the First Vatican Council in 1870 may be briefly stated as follows: The pope is infallible when he speaks ex cathedra -- that is, when, as pastor and teacher of all Christians, he defines, in virtue of his supreme apostolic authority, a doctrine concerning faith or morals to be held by the universal Church. Hence, there are four conditions for the exercise of papal infallibility, which we may summarize under the headings of (1) office, or subject of infallibility; (2) mode, or act of speaking infallibly; (3) content, or object of infallibility; and (4) recipient. (1) Office: The Pope must be speaking ex cathedra -- that is, from his position as supreme or universal pastor, not simply as a private theologian, or bishop of the Diocese of Rome, or sovereign of Vatican City State, or archbishop and metropolitan of the Roman Province, or primate of Italy, or patriarch of the West. (2) Mode: He must be defining a doctrine, not merely explaining, commenting, observing, exhorting or discussing, etc. In defining, he conclusively pronounces a doctrine with precision and certainty, enunciating it as a final and definitive judgment of truth, to the exclusion of alternatives, and the elimination of doubt. (3) Content: The doctrine must concern faith or morals. It need not be a revealed doctrine; it may be something already known by human reason -- for example, a point of the natural law such as the evil of murder, theft, etc. (4) Recipient: It must be addressed to all the Church, not merely one segment of her. In a way, (4) can be subsumed into (1), for a pope necessarily addresses all the faithful when he teaches by virtue of his supreme office. Reducing these conditions to the first three only, the Second Vatican Council summarizes the doctrine: "The Roman Pontiff, head of the college of bishops, enjoys this infallibility in virtue of his office, when, as supreme pastor and teacher of all the faithful [1] -- who confirms his brethren in the faith -- he proclaims by a definitive act [2] a doctrine concerning faith or morals [3]" (Lumen Gentium, Dogmatic Constitution on the Church, no 25). Canon 749.1 adopts this very text. In order to express the doctrine in a way that helps prevent misunderstanding, rather than say the pope is infallible (which can sound as if he always is), we should say that the pope can speak infallibly. Strictly speaking, a doctrine is not itself "infallible"; a doctrine is either true or false. Infallibility belongs to persons; by extension, we apply the word "infallible" to the doctrines they enunciate. The pope is never considered infallible in his personal or private views. Since the middle of the 19th century, only two ex cathedra pronouncements have been made in the Roman Catholic church: the definition of the dogma of the Immaculate Conception in 1854 by Pope Pius IX, and the definition of the Assumption of the Virgin in 1950 by Pope Pius XII."