Catholicisation refers mainly to the conversion of adherents of other religions into Catholicism, and the system of expanding Catholic influence in politics. Catholicisation was a policy of the Holy See through the Papal States, Holy Roman Empire, Habsburg monarchy, etc. Sometimes this process is referred to as re-Catholicization although in many cases Catholicized people had never been Catholics before.
The term is also used for the communion of Eastern Christian churches into the Catholic Church; the Eastern Catholic Churches that follow the Byzantine, Alexandrian, Armenian, East Syrian, and West Syrian Rites, as opposed to the Latin Church.