Protestant Reformation: A religious movement in Europe during the 16th century led by reformers like Martin Luther that challenged certain doctrines and practices of the Catholic Church.
Counter-Reformation: A response by the Catholic Church aimed at countering Protestantism through reforms within its own structure and reaffirmation of traditional beliefs.
Great Schism: Also known as the East-West Schism, it refers to the split between Eastern Orthodox Christianity (Byzantine Empire) and Western Christianity (Roman Catholic Church) in 1054.