The term Rasūl Allāh is often translated as "Messenger of God". Both terms "Messenger" and "God" are poor translations.
I avoid the term "God" as much as possible because I don't believe in the secular-Christian idea of "God" at all. Instead I've concluded from assessing reality that a single uncaused, unlimited cause of the universe exists... and from the Qur'ān that this cause is Allāh - unique from anything that they associate with Him. I refuse to associate Allāh with the confused and contradictory modern concept that is "God".
Likewise, I very much doubt you will find a single word that adequately translates the concept that is rasūl Allāh - because language evolves and develops around realities and there is no concept or reality of rasūl Allāh in western civilisational discourse. Ibn Taymiyyah argued Rasul refers to one sent to a hostile nation who rejects God in contrast to Nabi which refers to one sent to a receptive nation.
So I can't imagine there will be any one single word for it. Instead only a group of words together can give some indication of who Muhammad ibn Abdullah (saw) was.
The following seven words combined may help in conceptualising the meaning of rasūl in English:
1. Apostle
2. Institutor
3. Ambassador
4. Statesman
5. Politician
6. Law Receiver
7. Law Conveyor
Separately, he was already an amazing reformer, counsellor, companion, mentor, teacher, leader, guide and a Prophet before he became a rasūl.
