Peritus (Latin for "expert") is the title given to Roman Catholic theologians who are present to give advice at an ecumenical council. At the most recent council, the Second Vatican Council, some periti (the plural form) accompanied individual bishops or groups of bishops from various countries. Others were formally appointed as advisers to the whole Council.
Father Joseph Ratzinger, formerly Pope Benedict XVI, served as peritus to Cardinal Josef Frings, Archbishop of Cologne, Germany, while Hans Küng was a peritus for the Council, rather than for an individual Bishop. The influential German theologian Father Karl Rahner S.J. served as peritus to Cardinal Franz König of Vienna. John Henry Newman refused an invitation to be a peritus at the First Vatican Council.
The periti often advocated ideas of reform in the Church and were
often at the center of debates with some of the more traditional
scholars from the Coetus Internationalis Patrum.