Ar-Rabb is the owner who has full authority over his property. Ar-Rabb, linguistically means, the master or the one who has the authority to lead. All of these meanings are correct for Allah. When it is alone, the word Rabb is used only for Allah. As for other than Allah, it can be used to say Rabb Ad-Dar, the master of such and such object. Further, it was reported that Ar-Rabb is Allah’s Greatest Name.
Al-‘Alamin is plural for ‘Alam, which encompasses everything in existence except Allah. The word ‘Alam is itself a plural word, having no singular form. The ‘Alamin are different creations that exist in the heavens and the earth, on land and at sea. Every generation of creation is called an ‘Alam. Al-Farra and Abu Ubayd said, ” ‘Alam includes all that has a mind, the Jinns, mankind, the angels and the devils, but not the animals.” Also, Zayd bin Aslam and Abu Muhaysin said, `Alam includes all that Allah has created with a soul.” Further, Qatadah said about,
رَبِّ الْعَـلَمِينَ
(The Lord of the ‘Alamin), “Every type of creation is an ‘Alam.” Az-Zajjaj also said, “Alam encompasses everything that Allah created, in this life and in the Hereafter.” Al-Qurtubi commented, “This is the correct meaning, that the `Alam encompasses everything that Allah created in both worlds. Similarly, Allah said,
قَالَ فِرْعَوْنُ وَمَا رَبُّ الْعَـلَمِينَ – قَالَ رَبُّ السَّمَـوَتِ وَالاٌّرْضِ وَمَا بَيْنَهُمَآ إِن كُنتُمْ مُّوقِنِينَ
Fir`awn said: “And what is the Lord of the `Alamin” Musa said: “The Lord of the heavens and the earth, and all that is between them, if you seek to be convinced with certainty” (Ash-Shu’araa 23-24).
The creation is called ‘Alam is derived from ‘Alamah, that is because it is a sign testifying to the existence of its Creator and to His Oneness.”