Islam and Muslim have the same letter root (s,l,m) in Arabic. Islam means “submission to the truth” and Muslim means “one who submits to the truth”. The best definition for both words are mentioned in Quran.
Surah 3. Al-i'Imran, Ayah 83
أَفَغَيْرَ دِينِ اللَّهِ يَبْغُونَ وَلَهُ أَسْلَمَ مَنْ فِي السَّمَاوَاتِ وَالْأَرْضِ طَوْعًا وَكَرْهًا وَإِلَيْهِ يُرْجَعُونَ
Malik : Are they looking for a religion other than the Deen (religion and Way of Life) of Allah knowing well that everything in the heavens and in the earth, willingly or unwillingly, has submitted to Him? And to Him they shall all return.
In this verse, the holy Quran explains the broad meaning of Islam and says:
Everything in the heavens and in the earth is Muslim, meaning that everything submits to Him, but some groups willingly submit to the laws of God and some others unwillingly submit to Him.
Note that God has two kinds of commandments; one kind is natural and supernatural laws that control the whole universe with all creatures in it... all creatures have to obey them, if they disobey them, they will be destroyed. This is one kind of submission to God’s commands. Therefore, the sunlight that shines on the sea, water vapor that rises from the sea, the clouds that come together, the rain drops that fall from the sky, the trees that grow and the flowers that bloom,…. all of them are Muslims, because each submit to the creation laws.
But, there is another kind of God’s commands that submission to them is optional, and only true believers willingly submit to them. Of course, disobeying them lead to destruction, too.
From what was said, it is concluded that “ everything” in the sentence “everything in the heavens and in the earth”, refers to both groups, those who willingly submit to God (they are true believers) and those who unwillingly submit to God ( they are unwise creatures and disbelievers).
Reference: The interpretation of Surah 3. Al-i'Imran, Ayah 83, by Sayed Saeed Hosseini