The root of the `ar-rabb’ is `rabbaa-yarubbu’ which means to bring up something from stage to stage, up to the state of completion. It is said: “He brought him up,” and “They raised him up.”
“Ar- Rabb” is not used for anyone besides Him, except when it is attached in a specific sense, as is said: “rabbud-daar” (master of the house) and `Rabbul-faras’ (owner of the horse), such as His, the Most High’s, saying while narrating about Yusuf (alaihis salaam): “Mention me to your lord (i.e. your king).” But Shaytaan made him forget to mention it to his Lord. Yusuf 12:42
And the saying of the Most High: “As for one of you, he (as a servant) will pour out wine for his lord (king or master) to drink.” Yusuf 12:41
He (sallallaahu ‘alaihi wa sallam) said regarding the lost camel: “until its `rabb’ (owner) finds it.” (Reported by Bukhaari and Muslim)
So from this it becomes clear that `ar-rabb’ is applicable to Allaah in the definite form e.g. Ar-rabb (the Lord) or Rabbul-`Aalameen (Lord of the ‘Aalameen) or Rabbun-Naas (Lord of the People).