Vatican I
Definition: Vatican I refers to the First Vatican Council, which was held from 1869 to 1870. It was an important meeting of Catholic Church leaders in the Vatican City (the headquarters of the Catholic Church) that made significant decisions about church teachings. One of the main outcomes of this council was the declaration that the Pope is infallible, meaning that when the Pope speaks on matters of faith and morals in his official capacity (a situation known as "ex cathedra"), he is considered to be without error.
Vatican I was a crucial moment in the history of the Catholic Church, emphasizing the Pope's authority.