Mu'min is an Arabic Islamic term, frequently referenced in the Quran, meaning beliver and denotes a Muslim that has complete submission to the will of God (Allah), and has faith firmly established in his heart.

In the Quran it is stated:

 (Rodwell 49:14) The Arabs of the desert say, "We believe." (tu/minoo) Say thou: Ye believe not; but rather say, "We profess Islam;" (aslamna) for the faith (al-eemanu) hath not yet found its way into your hearts. But if ye obey God and His Apostle, he will not allow you to lose any of your actions: for God is Indulgent, Merciful. 

Also:

 (Shakir 4:136) O you who believe! believe in Allah and His Messenger and the Book which He has revealed to His Messenger and the Book which He revealed before; and whoever disbelieves in Allah and His angels and His messengers and the last day, he indeed strays off into a remote error. 

The verse addresses the believers, exhorting them to belive, implying multiple stages of belief.