09.09.07
History of Islam - Meaning of Caliph
According to DrB:
The follower of Muhammad was called the “caliph”, which comes from the Arabic word khalifa, which means viceregent. The first four caliphs and the Umayyad caliphs took the full title, which was “Viceregent/Successor to the Prophet of God” or “Khalifat Rasul Allah“.Later on as the Abbasid leaders became the most powerful dynasty, its charismatic rulers expunged the title “rasul” and simply called themselves “Khalifat Allah” or The Viceregent of God!