Caliph means successor. The Caliph is the chief Muslim civil and religious ruler. There are four Kulafa al-rashidoon/righteous caliphs and they are Abu Bakr, Ummar, Uthman, and Ali.
The Caliph (Arabic: خليفة ḫalīfah/khalīfah) is the head of state in a Caliphate, and the title for the ruler of the Islamic Ummah, an Islamic community ruled by the Shari'ah. It is a transcribed version of the Arabic word خليفة Khalīfah which means "successor" or "representative". Following Muhammad's death in 632, the early leaders of the Muslim nation were called "Khalifat Rasul Allah", the political successors to the messenger of God (referring to Muhammad). Some academics prefer to transliterate the term as Khalīfah. A Calipha is the wife or widow of a caliph or (rarely) female caliph, there was one known instance in history that a calipha was regent and ruler of a Caliphate, Sitt al-Mulk was regent of the Fatimid Caliphate from 1221 to 1223.