Which title is reserved for Muhammad's successors?

Caliph, Arabic khalīfah (“successor”), ruler of the Muslim community. When the Prophet Muhammad died (632 CE), Abū Bakr succeeded to his political and administrative functions as khalīfah rasūl Allāh, “successor of the Messenger of God,” but it was probably under ʿUmar ibn al-Khaṭṭāb, the second caliph, that the term caliph came into use as a title of the civil and religious head of the Muslim state.