In Catholic theology, the Latin phrase ex cathedra, literally meaning "from the chair", refers to a teaching by the pope that is considered to be made with the intention of invoking infallibility.
The "chair" referred to is not a literal chair, but refers metaphorically to the pope's position, or office, as the official teacher of Catholic doctrine: the chair was the symbol of the teacher in the ancient world, and bishops to this day have a cathedra, a seat or throne, as a symbol of their teaching and governing authority. The pope is said to occupy the "chair of Peter", as Catholics hold that among the apostles Peter had a special role as the preserver of unity, so the pope as successor of Peter holds the role of spokesman for the whole church among the bishops, the successors as a group of the apostles.
Popes seldom use their power of infallibility, but rely on the notion that the Church allows the office of the pope to be the ruling agent in deciding what will be accepted as formal beliefs in the church."Since the solemn declaration of Papal Infallibility by Vatican I on July 18, 1870, this power has been used only once ex cathedra: in 1950 when Pope Pius XII defined the Assumption of Mary as being an article of faith for Roman Catholics. Thus, for example, the encyclical Humanae Vitae, issued in 1968 by Pope Paul VI is not considered to be infallible.
So pretty much, if something is declared EX CATHEDRA, everyone would read about it in the papers and such. It's really not that big of a deal in general use.