The Meaning of the Two Terms Rabb and Ilah
It has now been shown multiple times that the word ilah fundamentally means “object of worship.” This can be clearly seen from dozens of Qur'anic verses.
However, since worthiness of worship is based on divine attributes, the word ilah/ object of worship is closely connected to the word rabb/ Lord (plural: arbab). The one who is truly worthy of worship deserves worship because He is the Creator, Sustainer, and Giver of life, or the All-Knowing and All-Powerful.
In other words: The ilah who truly deserves worship due to His attributes – and not merely one who is claimed to be worthy of worship – is necessarily also the true rabb/Lord.
When two words have such closely related meanings, it is common in Arabic for both meanings to be combined into one word. More precisely, it is not unusual in the Arabic language that – due to the closeness in meaning – one term is used while also implying the meaning of the other term. Thus, one word is spoken, but both meanings are intended.
Islamic scholars have said regarding such words:
إذا اجتمعا تفرَّقا، وإذا تفرَّقا اجتمعا
When the two words come together in one place, their meanings diverge. And when the two words are separated, their meanings come together.
They meant by this that each word essentially has its own meaning. However, when one word stands alone – and the context does not clearly indicate otherwise – the meaning of the other word is included within the meaning of the first. But when both terms are explicitly mentioned together in the same sentence, this context indicates that each specific meaning is intended separately.
The same applies, for example, to the words iman and Islam. Iman originally refers to what takes place internally, while Islam describes the external. As mentioned, this phenomenon is not uncommon in the Arabic language.