It is a verb as well as a noun.
It's meaning:
1.surrender to the Almighty God
2.submission to his will
3.Obey his commandments
4.Sincere worship only for him
5.Peace in mind, heart, and soul
Anyone, or anything, that lives according to God's will, is called "Muslim: the one who do the action called Islam". Example, the moon, the star, some human being, etc.
Not sure, if my answer is okey for your question.