It is pathetic that there is not authentic Arabic to English translation available online. I came across a few closer definitions. This wiki link translates Rub as
rub is an Arabic name which means the master & the owner of something.
It's used to talk about God as the Lord of everything. It also has
other usages & it's NOT used as a name of a person.
I agree with this because "Rub" does not really mean God but it can. In fact for non-arabic speakers (like myself) it is always taken as God but that definition is not true. Even Google translate it to God
Babla dictionary does not lists its actual meaning but gives a few examples of it. Eg.
رَبُّ is not an actual word for God although it can be translated into God as one of it meaning. The meaning of "Rub" is masters or Lord. It can be used for God as well as he is the true master and lord.
ربّ العمل -> employer
"وقل ربِّ زدني علمًا" -> And say, "My Lord, increase me in knowledge."
"وقل ربِّ زدني علمًا". -> But say, "O my Lord! advance me in knowledge."
As you can see, one use of it has meaning of "Employer", the other usages means "God". Therefor "Rub" is not a specific word for God but more of Lord, Master, owner. Since God is the ultimate Master, Lord, this word can be used for God. In fact I have seen the word "Owner" in my native language used for God as well.
In short "God" in English is Translated to "Allah". "Lord" is translated into Rub. This google translation may be helpful. A native arabic speaker may provide a more authoritative answer on this.