A Muslim (Arabic: مسلم, literally: he who submits) is an adherent of Islam. The word stems from the prefix mu- and the noun slim . Which is similar to the English use of suffix submit -er, build -er. The noun 'slim' has as its three root words S-L-M, which are also the three root words of Islam which has an underlying meaning of submission or surrender.
So a Muslim is someone who submits to Allah, and Islam is the religion of submission to Allah.