مؤمن
Muʾmin
A man who has iman, faith; the plural is muʾminūn.
What it means
Muʾmin is a man who holds īmān, faith, meaning belief in God, His angels, His books, His messengers, the Last Day and the divine decree. The Quran says, “The believers are only those who believe in God and His Messenger and then do not doubt” (49:15). Al-Muʾmin is also one of the names of God (59:23). The Quran distinguishes it from muslim. In one passage the desert Arabs say, “We believe”, and the Quran replies that they should say, “We have submitted”, since faith has not yet entered their hearts (49:14). The word muʾmin thus names a faith held inwardly and shown in action.
Often misunderstood as
Translating it as “believer” is acceptable, but “faithful” alone can lose that the word means a person with settled inner faith as well as outward practice.
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 23, listed as MU'MIN. The definition is restated here.