What Is A 'Muslim'?
One who believes and affirms that there is only One God, that Muhammad (pbuh) is His Last and Final Prophet and Messenger, and that there is to be a Day of Judgement, is called a Muslim.
Muslims are the followers of the religion of Islam, i.e. those who submit to the will of Allah as explained in the Qur'an and the sayings of Prophet Muhammad (AS), the Messenger of Islam. The Muslim population today is about 1.2 billion and is spread over a vast range of races, nationalities, and cultures.
Approximately 18% of the Muslims live in the Arab world, but the majority live in Asia and Africa. The country that has the largest Muslim population is Indonesia, and significant Muslim minorities exist in Russia, China, and Europe as well as North and South America. The Muslim population of the United States is estimated to be around 6 million.
Allamah Sayyid Sa'eed Akhtar Rizvi (1987) Some Terminologies Explained. Available at: https://www.al-islam.org/what-a-muslim-should-know-and-believe-akhtar-rizvi/some-terminologies-explained#muslim
Al-Qazwini, S. M.Introduction. Available at: https://www.al-islam.org/invitation-islam-sayyid-moustafa-al-qazwini/introduction#what-islam