Amir-ul-Momineen

Amir al-Mu'minin (Arabic: أمير المؤمنين‎‎), usually translated "Commander of the Faithful" or "Leader of the Faithful", is the Arabic style of some Caliphs and other independent sovereign Muslim rulers that claim legitimacy from a community of Muslims. It has been claimed as the title of rulers in Muslim countries and empires and is still used for some Muslim leaders.

The use of the title does not necessarily signify a claim to caliphate as it is usually taken to be, but described a certain form of activist leadership which may have been attached to a caliph but also could signify a level of authority beneath that. The Ottoman sultans, in particular, made scant use of it. Moreover, the term was used by men who made no claim to be caliphs.

Amir al-Mu'minin is latinized as Miramolinus, hence Italian Miramolino, French Miramolin, Spanish Miramolín and Portuguese Miramolim, in Byzantine Greek: ἀμερμουμνῆς amermoumnês.

It is also translated as "Prince of the believers" since "Amir" or "Emir" is also used as a princely title in states ruled by the royalty or monarchies.

Sunnis generally consider Umar the first person given the title. According to the Islamic scholar as-Suyuti (1445–1505):

Umar ibn Abd al-Aziz asked Abu Bakr ibn Sulayman ibn Abi Hathamah what was the reason that it used to be written, "From the Khalifah of the Messenger of Allah, may Allah bless him and grant him peace," in the time of Abu Bakr, then later Umar used to write at first, "From the Khalifah of Abu Bakr."? Then who was the first to write, "From the Amir al-Muminin (the Commander of the Believers)"?

...
Wikipedia