In his Peak of Eloquence Imam Ali (as) give a most wonderful definition of the word Islam. Though already produced on this site, amongst many other immaculate sayings, I believe this one in particular deserves another look at.
Imam Ali (as):
I will define Islam for you in a way that nobody dared do it before me. Islam means obedience to Allah, obedience to Allah means having sincere faith in Him, such a faith means to believe in His Power, belief in His Power means recognizing and accepting His Majesty, acceptance of His Majesty means fulfilling the obligations laid down by Him and fulfillment of obligations means actions.
Nahjul Balagha: section of the sayings of the Imam